1.13.0 - system prune vs. images -f dangling=true: different behaviour

[RRAlex]

Description
docker system prune -f is supposed to remove dangling images but images tagged <none> aren't necessarily removed.
Meanwhile, docker rmi $(docker images -f dangling=true) will remove more images.
Something is not coherent in one of those?

Steps to reproduce the issue:

1. Pull an image by a digest (docker pull image@sha256:...)
2. docker system prune -f
3. docker rmi $(docker images -f dangling=true)

Describe the results you received:
3rd command above still deletes an image.

Describe the results you expected:
Prune should have done all the work before the last command.

Additional information you deem important (e.g. issue happens only occasionally):

Output of docker version:

Client:
 Version:      1.13.0
 API version:  1.25
 Go version:   go1.7.3
 Git commit:   49bf474
 Built:        Tue Jan 17 09:58:26 2017
 OS/Arch:      linux/amd64

Server:
 Version:      1.13.0
 API version:  1.25 (minimum version 1.12)
 Go version:   go1.7.3
 Git commit:   49bf474
 Built:        Tue Jan 17 09:58:26 2017
 OS/Arch:      linux/amd64
 Experimental: false

Output of docker info:

 Running: 0
 Paused: 0
 Stopped: 0
Images: 23
Server Version: 1.13.0
Storage Driver: aufs
 Root Dir: /var/lib/docker/aufs
 Backing Filesystem: extfs
 Dirs: 31
 Dirperm1 Supported: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins: 
 Volume: local
 Network: bridge host macvlan null overlay
Swarm: inactive
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: 03e5862ec0d8d3b3f750e19fca3ee367e13c090e
runc version: 2f7393a47307a16f8cee44a37b262e8b81021e3e
init version: 949e6fa
Security Options:
 apparmor
 seccomp
  Profile: default
Kernel Version: 4.4.0-59-generic
Operating System: Linux Mint 18
OSType: linux
Architecture: x86_64
CPUs: 8
Total Memory: 30.99 GiB
Name: octet
ID: Z6SL:RSSC:M7BQ:NXJX:KHVZ:7IAZ:TIO2:G2QP:HKVC:NM76:ZWOH:KMVC
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): false
Registry: https://index.docker.io/v1/
WARNING: No swap limit support
Experimental: false
Insecure Registries:
 127.0.0.0/8
Live Restore Enabled: false

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

[AlekSi]

I would like docker system prune to remove untagged images. If this is not going to be a default behavior, can we have a flag for that? It's useful after pulling newest latest images.

[mlaventure]

@AlekSi try using the -a flag. By default it was decided not to remove images without a container attached to them as people may just be using the machine as a build machine and doing so would erase their cache.

[AlekSi]

I use Docker with Drone CI. I don't want to remove, for example, latest mysql:5.7 even if it doesn't have a container, but I want to remove previous versions of this tag. -a would remove the latest version, without this flag previous versions will not be removed.

I wonder what images docker system prune without -a removes?

This issue is about a difference in behavior and definition of the word "dangling". If you want to keep it, then I think:

1. Help message should be updated to describe what would be removed.
2. Some flag should be added to remove all untagged images.

[mlaventure]

@AlekSi oh, I didn't understand your first comment it seems.

#30330 should fix system prune so it also removes untagged images hence matching dangling definition in docker image -f dangling=true