Enhance Docker image summary with structured inspect output

[goruha]

what

• Replace raw JSON metadata dump in step summary with a rich, structured markdown summary
• Add new summary input (default true) to control step summary generation independently from inspect
• Add new inspect output exposing the full docker inspect JSON
• Change metadata output to use docker/build-push-action metadata directly instead of a separate shell step
• Remove the old Get Metadata step entirely
• Disable inspect in test workflows (test-docker-build.yml, test-docker-build-multi-platform.yml)
• Control DOCKER_BUILD_SUMMARY env var based on inspect and summary inputs

why

• The previous raw JSON summary was hard to read in GitHub Actions job summaries
• The new format surfaces key image details at a glance: size, architecture, OS, license, tag, digest, revision, source, runtime config, env vars, labels, and layers — each in collapsible sections
• Separating summary from inspect gives users finer control over what gets generated

references

• https://github.com/docker/build-push-action

[github-actions]

These changes were released in v3.0.0.