feat: use pre-built Docker image from GHCR

[jankal]

Summary

• Add a release workflow (.github/workflows/release.yml) that builds and pushes the action's Docker image to GHCR on every published GitHub release
• Update action.yaml to pull the pre-built image (docker://ghcr.io/mittwald/deploy-container-action:v1) instead of building from the Dockerfile

This reduces action startup time from ~30–60s (Go compile on every run) to ~2–5s (pull a ~20 MB Alpine image).

How it works

On every published release (e.g. v1.0.2), the workflow builds the Docker image and pushes it with semver tags:

Tag	Example	Purpose
v{{major}}	v1	Rolling major — used by action.yaml
v{{major}}.{{minor}}	v1.0	Rolling minor
v{{version}}	v1.0.2	Exact version
sha-{{sha}}	sha-abc1234	Commit pinning

Manual steps required after merge

1. Create a GitHub release to trigger the first image build
2. Make the GHCR package public so users can pull without authentication:
   • Go to https://github.com/orgs/mittwald/packages
   • Select deploy-container-action
   • Package settings → Danger Zone → Change package visibility → Public
3. Force-update the v1 git tag to include the new action.yaml:

   git tag -fa v1 -m "Release v1 – use pre-built image"
   git push origin v1 --force

Test plan

• Merge this PR
• Create a release (e.g. v1.0.3) and verify the image appears at ghcr.io/mittwald/deploy-container-action
• Make the package public
• Update the v1 tag
• Test the action in a consumer workflow and confirm it pulls the pre-built image instead of building

🤖 Generated with Claude Code