Blanking a parent image's ENTRYPOINT busts the child's cache

[shargan]

Description
If a parent docker image defines ENTRYPOINT and a child blanks it out (ENTRYPOINT []), that line in the child busts the cache.

Steps to reproduce the issue:
Create three Dockerfiles:

bugreport$ cat Dockerfile.parent
FROM ubuntu:16.04

ENTRYPOINT ["/bin/bash"]
bugreport$ cat Dockerfile.child.empty
FROM parent

RUN echo "This remains cached"
ENTRYPOINT []
RUN echo "This does not get cached"
bugreport$ cat Dockerfile.child.redefined
FROM parent

RUN echo "This remains cached"
ENTRYPOINT ["/bin/ls"]
RUN echo "So does this"

Now build the parent:

bugreport$ docker build -t parent -f Dockerfile.parent .
Sending build context to Docker daemon  4.096kB
Step 1/2 : FROM ubuntu:16.04
 ---> 7aa3602ab41e
Step 2/2 : ENTRYPOINT ["/bin/bash"]
 ---> Running in ccb2deaac166
Removing intermediate container ccb2deaac166
 ---> a16fa2461ffc
Successfully built a16fa2461ffc
Successfully tagged parent:latest

Describe the results you expected:
Repeatedly building child.redefined behaves exactly as expected. Each stage is cached and the final result is identical each timel: 00427e0e1d2e.

bugreport$ docker build -t child.redefined -f Dockerfile.child.redefined .
Sending build context to Docker daemon  4.096kB
Step 1/4 : FROM parent
 ---> a16fa2461ffc
Step 2/4 : RUN echo "This remains cached"
 ---> Running in 986158df8103
This remains cached
Removing intermediate container 986158df8103
 ---> 3f75db9409f7
Step 3/4 : ENTRYPOINT ["/bin/ls"]
 ---> Running in defe95cbcbd6
Removing intermediate container defe95cbcbd6
 ---> 5500f9c8717e
Step 4/4 : RUN echo "So does this"
 ---> Running in 9c8f27fa210d
So does this
Removing intermediate container 9c8f27fa210d
 ---> 00427e0e1d2e
Successfully built 00427e0e1d2e
Successfully tagged child.redefined:latest

bugreport$ docker build -t child.redefined -f Dockerfile.child.redefined .
Sending build context to Docker daemon  4.096kB
Step 1/4 : FROM parent
 ---> a16fa2461ffc
Step 2/4 : RUN echo "This remains cached"
 ---> Using cache
 ---> 3f75db9409f7
Step 3/4 : ENTRYPOINT ["/bin/ls"]
 ---> Using cache
 ---> 5500f9c8717e
Step 4/4 : RUN echo "So does this"
 ---> Using cache
 ---> 00427e0e1d2e
Successfully built 00427e0e1d2e
Successfully tagged child.redefined:latest

Describe the results you received:
Repeatedly building child.empty shows that the cache busts at the ENTRYPOINT line. As a result, a new image is produced and tagged on every single build.

bugreport$ docker build -t child.empty -f Dockerfile.child.empty .
Sending build context to Docker daemon  4.096kB
Step 1/4 : FROM parent
 ---> a16fa2461ffc
Step 2/4 : RUN echo "This remains cached"
 ---> Using cache
 ---> 3f75db9409f7
Step 3/4 : ENTRYPOINT []
 ---> Running in 0ad4aefa6893
Removing intermediate container 0ad4aefa6893
 ---> 19d67ed46785
Step 4/4 : RUN echo "This does not get cached"
 ---> Running in 7db2a04106e4
This does not get cached
Removing intermediate container 7db2a04106e4
 ---> ccdea9f05e9d
Successfully built ccdea9f05e9d
Successfully tagged child.empty:latest

bugreport$ docker build -t child.empty -f Dockerfile.child.empty .
Sending build context to Docker daemon  4.096kB
Step 1/4 : FROM parent
 ---> a16fa2461ffc
Step 2/4 : RUN echo "This remains cached"
 ---> Using cache
 ---> 3f75db9409f7
Step 3/4 : ENTRYPOINT []
 ---> Running in 001c6f25cf8b
Removing intermediate container 001c6f25cf8b
 ---> 41312197b9d4
Step 4/4 : RUN echo "This does not get cached"
 ---> Running in 468b23e378c7
This does not get cached
Removing intermediate container 468b23e378c7
 ---> 2515835107b0
Successfully built 2515835107b0
Successfully tagged child.empty:latest

Since the dockerfile has not changed, each stage should be cached and an identical image produced/tagged at the end.

Output of docker version:

bugreport$ docker version
Client:
 Version:           18.06.0-ce
 API version:       1.38
 Go version:        go1.10.3
 Git commit:        0ffa825
 Built:             Wed Jul 18 19:05:26 2018
 OS/Arch:           darwin/amd64
 Experimental:      false

Server:
 Engine:
  Version:          18.06.0-ce
  API version:      1.38 (minimum version 1.12)
  Go version:       go1.10.3
  Git commit:       0ffa825
  Built:            Wed Jul 18 19:13:46 2018
  OS/Arch:          linux/amd64
  Experimental:     true

Output of docker info:

bugreport$ docker info
Containers: 0
 Running: 0
 Paused: 0
 Stopped: 0
Images: 119
Server Version: 18.06.0-ce
Storage Driver: overlay2
 Backing Filesystem: extfs
 Supports d_type: true
 Native Overlay Diff: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
 Volume: local
 Network: bridge host ipvlan macvlan null overlay
 Log: awslogs fluentd gcplogs gelf journald json-file logentries splunk syslog
Swarm: inactive
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: d64c661f1d51c48782c9cec8fda7604785f93587
runc version: 69663f0bd4b60df09991c08812a60108003fa340
init version: fec3683
Security Options:
 seccomp
  Profile: default
Kernel Version: 4.9.93-linuxkit-aufs
Operating System: Docker for Mac
OSType: linux
Architecture: x86_64
CPUs: 4
Total Memory: 1.952GiB
Name: linuxkit-025000000001
ID: ZPNR:CRR5:7M66:5RIA:U6BT:CRKN:EIVT:O7TZ:5X2E:P447:UFT6:KBXZ
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): true
 File Descriptors: 26
 Goroutines: 53
 System Time: 2018-08-15T01:18:23.5042913Z
 EventsListeners: 2
HTTP Proxy: gateway.docker.internal:3128
HTTPS Proxy: gateway.docker.internal:3129
Registry: https://index.docker.io/v1/
Labels:
Experimental: true
Insecure Registries:
 127.0.0.0/8
Live Restore Enabled: false

Additional environment details (AWS, VirtualBox, physical, etc.):

This is on a Macbook Pro running High Sierra 10.13.6.

I cannot reproduce the issue on an Ubuntu 16.04 host running 17.05.0-ce server and client.

[thomasbiddle]

I've run into this as well; I believe it was only when mine was set to [""] - [" "] and [] worked - but I'd have to reverify.

[Hafas]

This is on a Macbook Pro running High Sierra 10.13.6.
I cannot reproduce the issue on an Ubuntu 16.04 host running 17.05.0-ce server and client.

I can reproduce the issue on Ubuntu 18.04.3 and Docker 19.03.2

Simple Dockerfile:

FROM ubuntu

RUN echo "this gets cached"

ENTRYPOINT ["true"]

RUN echo "this gets cached too"

ENTRYPOINT []

RUN echo "this one doesn't get cached"

Output on first build:

Sending build context to Docker daemon  2.048kB
Step 1/6 : FROM ubuntu
 ---> a2a15febcdf3
Step 2/6 : RUN echo "this gets cached"
 ---> Running in ab0cc610bd9f
this gets cached
Removing intermediate container ab0cc610bd9f
 ---> 8022cc94e411
Step 3/6 : ENTRYPOINT ["true"]
 ---> Running in d8c3212d131e
Removing intermediate container d8c3212d131e
 ---> b8178335171a
Step 4/6 : RUN echo "this gets cached too"
 ---> Running in 79725d93d5c2
this gets cached too
Removing intermediate container 79725d93d5c2
 ---> 91cd779e3b13
Step 5/6 : ENTRYPOINT []
 ---> Running in f236288f83ec
Removing intermediate container f236288f83ec
 ---> 235b1973a8f3
Step 6/6 : RUN echo "this one doesn't get cached"
 ---> Running in e9477f805d8d
this one doesn't get cached
Removing intermediate container e9477f805d8d
 ---> 93909525e605
Successfully built 93909525e605

Output on 2nd and further builds:

Sending build context to Docker daemon  2.048kB
Step 1/6 : FROM ubuntu
 ---> a2a15febcdf3
Step 2/6 : RUN echo "this gets cached"
 ---> Using cache
 ---> 8022cc94e411
Step 3/6 : ENTRYPOINT ["true"]
 ---> Using cache
 ---> b8178335171a
Step 4/6 : RUN echo "this gets cached too"
 ---> Using cache
 ---> 91cd779e3b13
Step 5/6 : ENTRYPOINT []
 ---> Running in 01920ae6d559
Removing intermediate container 01920ae6d559
 ---> 5629755d105b
Step 6/6 : RUN echo "this one doesn't get cached"
 ---> Running in 1ec17e10beab
this one doesn't get cached
Removing intermediate container 1ec17e10beab
 ---> 0de98c21d7f9
Successfully built 0de98c21d7f9

Step 5 & 6 doesn't get cached

[tianon]

This was rediscovered/reconfirmed in #3465 (comment). 😞

[tianon]

$ docker version
Client: Docker Engine - Community
 Version:           20.10.0-beta1
 API version:       1.41
 Go version:        go1.13.15
 Git commit:        ac365d7
 Built:             Tue Oct 13 18:13:41 2020
 OS/Arch:           linux/amd64
 Context:           default
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          20.10.0-beta1
  API version:      1.41 (minimum version 1.12)
  Go version:       go1.13.15
  Git commit:       9c15e82
  Built:            Tue Oct 13 18:17:18 2020
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          v1.4.1
  GitCommit:        c623d1b36f09f8ef6536a057bd658b3aa8632828
 runc:
  Version:          1.0.0-rc92
  GitCommit:        ff819c7e9184c13b7c2607fe6c30ae19403a7aff
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

$ cat Dockerfile
FROM bash
ENTRYPOINT []
RUN echo never cached

$ docker build .
Sending build context to Docker daemon  2.048kB
Step 1/3 : FROM bash
latest: Pulling from library/bash
188c0c94c7c5: Pull complete 
94387ca39817: Pull complete 
efe7174943e6: Pull complete 
Digest: sha256:01fad26fa8ba21bce6e8c47222acfdb54649957f1e86d53a0c8e03360271abf6
Status: Downloaded newer image for bash:latest
 ---> 39a95ac32011
Step 2/3 : ENTRYPOINT []
 ---> Running in 849b813584b7
Removing intermediate container 849b813584b7
 ---> 83780b57402b
Step 3/3 : RUN echo never cached
 ---> Running in f67165121267
never cached
Removing intermediate container f67165121267
 ---> 129d3f56aaad
Successfully built 129d3f56aaad

$ docker build .
Sending build context to Docker daemon  2.048kB
Step 1/3 : FROM bash
 ---> 39a95ac32011
Step 2/3 : ENTRYPOINT []
 ---> Running in 893405f7299b
Removing intermediate container 893405f7299b
 ---> 5e34458be803
Step 3/3 : RUN echo never cached
 ---> Running in 026c08b3f84e
never cached
Removing intermediate container 026c08b3f84e
 ---> ceb1d3fed119
Successfully built ceb1d3fed119

[thaJeztah]

Yes, was able to reproduce; looks to be only with the classic builder, and not with buildkit;

mkdir repro-37643 && cd repro-37643

cat > Dockerfile -<<'EOF'
FROM bash
ENTRYPOINT []
RUN echo never cached
EOF

DOCKER_BUILDKIT=0 docker build .

Sending build context to Docker daemon  2.048kB
Step 1/3 : FROM bash
latest: Pulling from library/bash
188c0c94c7c5: Already exists
94387ca39817: Pull complete
efe7174943e6: Pull complete
Digest: sha256:01fad26fa8ba21bce6e8c47222acfdb54649957f1e86d53a0c8e03360271abf6
Status: Downloaded newer image for bash:latest
 ---> 39a95ac32011
Step 2/3 : ENTRYPOINT []
 ---> Running in 0bfdba8a93a9
Removing intermediate container 0bfdba8a93a9
 ---> 949b6885127b
Step 3/3 : RUN echo never cached
 ---> Running in b4cff1140a3c
never cached
Removing intermediate container b4cff1140a3c
 ---> 05b7507d3eda
Successfully built 05b7507d3eda

DOCKER_BUILDKIT=0 docker build .
Sending build context to Docker daemon  2.048kB
Step 1/3 : FROM bash
 ---> 39a95ac32011
Step 2/3 : ENTRYPOINT []
 ---> Running in 3f78aa541994
Removing intermediate container 3f78aa541994
 ---> 0b195faa28c7
Step 3/3 : RUN echo never cached
 ---> Running in 0d5ddecf31ee
never cached
Removing intermediate container 0d5ddecf31ee
 ---> 882bf67681da
Successfully built 882bf67681da

bash-5.0$ DOCKER_BUILDKIT=1 docker build .
[+] Building 1.1s (6/6) FINISHED
 => [internal] load .dockerignore                                                                0.2s
 => => transferring context: 2B                                                                  0.0s
 => [internal] load build definition from Dockerfile                                             0.1s
 => => transferring dockerfile: 88B                                                              0.0s
 => [internal] load metadata for docker.io/library/bash:latest                                   0.0s
 => [1/2] FROM docker.io/library/bash                                                            0.3s
 => => resolve docker.io/library/bash:latest                                                     0.0s
 => [2/2] RUN echo never cached                                                                  0.4s
 => exporting to image                                                                           0.1s
 => => exporting layers                                                                          0.1s
 => => writing image sha256:a1bb8d52d475aec5a3cc09b2ece0b40a881dac44512fa6b5530c05fe462359c6     0.0s

bash-5.0$ DOCKER_BUILDKIT=1 docker build .
[+] Building 0.2s (6/6) FINISHED
 => [internal] load .dockerignore                                                                0.1s
 => => transferring context: 2B                                                                  0.0s
 => [internal] load build definition from Dockerfile                                             0.2s
 => => transferring dockerfile: 36B                                                              0.0s
 => [internal] load metadata for docker.io/library/bash:latest                                   0.0s
 => [1/2] FROM docker.io/library/bash                                                            0.0s
 => CACHED [2/2] RUN echo never cached                                                           0.0s
 => exporting to image                                                                           0.0s
 => => exporting layers                                                                          0.0s
 => => writing image sha256:a1bb8d52d475aec5a3cc09b2ece0b40a881dac44512fa6b5530c05fe462359c6     0.0s