feat: Add OCI labels for git commit and branch to docker images

[KatyaRyazantseva]

Adds OCI labels to both Dockerfile and Dockerfile.prebuilt for better image traceability:

• org.opencontainers.image.ref.name - git branch
• org.opencontainers.image.revision - git commit

Labels are set via build args with unknown as default, so existing builds continue to work.

Updated README with build example for publishing to registries.

Build with labels:

docker build -f Dockerfile.prebuilt \
  --build-arg GIT_COMMIT=$(git rev-parse HEAD) \
  --build-arg GIT_BRANCH=$(git rev-parse --abbrev-ref HEAD) \
  -t zeam:local .

Related to #91

[KatyaRyazantseva]

@g11tech ready for review

[g11tech]

you will need to modify our docker image release workflow as well cc @noopur23

cc @gballet any opinions here?

[gballet]

My suggestion is that we also use this inside the build.zig to tag the executable itself, using -Dversion=$GIT_COMMIT-$GIT_BRANCH when building in auto-release.yml, but I'll leave it to you to decide if this is really interesting.