podman build --squash also squashes layers of base container

[mlschroe]

Description

podman build --squash squashes all layers into a single layer, whereas docker build --squash only squashes the new layers but keeps the layers from the base container. Is this difference intentional? It IMHO limits the usefulness of the --squash option quite a bit.

Steps to reproduce the issue:

1. podman build --squash

Describe the results you received:
The created container has only one layer.

Describe the results you expected:
The layers of the base container should be kept intact.

Output of rpm -q buildah or apt list buildah:

buildah-1.5-1.1.x86_64

Output of buildah version:

Version:         1.5
Go Version:      go1.11.1
Image Spec:      1.0.0
Runtime Spec:    1.0.0
CNI Spec:        0.4.0
libcni Version:  v0.7.0-alpha1
Git Commit:      87239ae7046ea3a25f644cd0789b3d6678dc144d
Built:           Fri Nov 23 12:00:00 2018
OS/Arch:         linux/amd64

Output of podman version if reporting a podman build issue:

Version:       0.11.1.1
Go Version:    go1.11.1
OS/Arch:       linux/amd64

[TomSweeneyRedHat]

@rhatdan @umohnani8 WDYT?

[rhatdan]

Definitely not intentional, we want to match Docker behaviour.

[pkubatrh]

Also hit this today while trying to see what to use instead of docker-squash in our workflow.

[vrothberg]

This one must have slipped over Christmas. Thanks for the report and the ping. We'll have a look.

[TomSweeneyRedHat]

@pkubatrh or @mlschroe do either of you have a Dockerfile that reproduces this handy? If not, I'll try to dig one up.

[TomSweeneyRedHat]

Never mind on the Dockerfile, I found a nice chunk on https://stackoverflow.com/questions/42786991/docker-squash-behavior

[rhatdan]

@nalind @TomSweeneyRedHat Whats the scoop on this one? Does some of the stuff @nalind is working on fix it?

[TomSweeneyRedHat]

Doesn't appear that @nanlind's changes fixed this, but I'll let him correct me.

[rhatdan]

@vrothberg @nalind @TomSweeneyRedHat Any progress on this?

[TomSweeneyRedHat]

Not that I know of, sigh, slipped off the radar.

[MaciejKucia]

This seems a requirements problem since original committer has assumed that all layers shall be squashed.

c806e6e#diff-b9ec73f876c8864b1fbdbf9fd96b4690

Reusing base layers makes a lot of sense, especially when dealing with many images based on common FROM (for example RedHat UBI).

[nalind]

Agreed, this is essentially a mislabeling based on a misread of what squash was supposed to be doing. A squashed result from docker build is equivalent to our --layers=false, but there's not really a good time to rename flags in the CLI and in the APIs.