For example, consider pushing 5 commits, one of which contains a
dockerfile change. This will trigger a CI pipeline for the
containers. Now consider you do some more work on the branch and push 3
further commits, so you now have a branch of 8 commits. For the second
push GitLab will only look at the 3 most recent commits, the other 5
were already present. Thus GitLab will not realize that the branch has
dockerfile changes that need to trigger the container build.
This can cause real world problems:
- Push 5 commits to branch "foo", including a dockerfile change
=> rebuilds the container images with content from "foo"
=> build jobs runs against containers from "foo"
- Refresh your master branch with latest upstream master
=> rebuilds the container images with content from "master"
=> build jobs runs against containers from "master"
- Push 3 more commits to branch "foo", with no dockerfile change
=> no container rebuild triggers
=> build jobs runs against containers from "master"
The "changes" conditional in gitlab is OK, *provided* your build
jobs are not relying on any external state from previous builds.
This is NOT the case in QEMU, because we are building container
images and these are cached. This is a scenario in which the
"changes" conditional is not usuable.
The only other way to avoid this problem would be to use the git
branch name as the container image tag, instead of always using
"latest".