--rm=false does not keep intermediate containers as expected

[bkomraz1]

Issue description

DOCKER_BUILDKIT=1 docker build --rm=false . does not keep intermediate containers as docker build --rm=false . keeps

# Dockerfile
FROM alpine:latest
LABEL debug=true

FROM alpine:latest

Steps to reproduce the issue

1. docker system prune -f
2. DOCKER_BUILDKIT=1 docker build --rm=false .
3. docker ps -a -f label=debug=true

What's the expected result?

Step 1/3 : FROM alpine:latest
 ---> f70734b6a266
Step 2/3 : LABEL debug=true
 ---> Running in 773f64e4fbbe
 ---> d941f8903851
Step 3/3 : FROM alpine:latest
 ---> f70734b6a266
Successfully built f70734b6a266
CONTAINER ID        IMAGE               COMMAND                  CREATED                  STATUS              PORTS               NAMES
773f64e4fbbe        f70734b6a266        "/bin/sh -c '#(nop) …"   Less than a second ago   Created                                 friendly_heyrovsky

What's the actual result?

[+] Building 0.2s (5/5) FINISHED
 => [internal] load build definition from Dockerfile                                                                                         0.1s
 => => transferring dockerfile: 98B                                                                                                          0.1s
 => [internal] load .dockerignore                                                                                                            0.1s
 => => transferring context: 2B                                                                                                              0.1s
 => [internal] load metadata for docker.io/library/alpine:latest                                                                             0.0s
 => CACHED [stage-1 1/1] FROM docker.io/library/alpine:latest                                                                                0.0s
 => exporting to image                                                                                                                       0.0s
 => => exporting layers                                                                                                                      0.0s
 => => writing image sha256:db489feeb4704a920c415104748a2ceef4e8e97882957f8e279813bc362a040f                                                 0.0s
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

[tonistiigi]

#1472

[alexanderkjeldaas]

This issue should not be closed in favor of #1472 as #1472 only addresses one use-case.

Let me give a different use-case which belongs to this ticket: pushing intermediate images using labels.

See old thread in the docker forum here, SO question here etc.

Running docker multiple times is typically not a solution as this can be inside a complex CI/CD pipeline where a gazillion parameters are given to docker.

The previous recommended approach was using the LABEL command and then filter images based on this. This is a robust approach that composes well with any build system, unlike approaches requiring multiple invocations.

[TBBle]

Intermediate images are quite different from intermediate containers.

This ticket was asking about the latter, and was closed because (I assume) #1472 contains the discussion about why BuildKit doesn't have the sort of intermediate containers that docker build had, and is hence infeasible, in the second and third paragraphs.

The use-case of pushing multiple targets from a build as individual images is interesting, but that seems like an unrelated feature request for this ticket. (Also only tangentially related to #1472: whatever debugging approach is implemented, and generating multiple images in a single build, might have some implementation details in common, but those aspects already exist in BuildKit, so the remaining effort would be minimally-shared.)