Add docker build workflow

[gaborgsomogyi]

Summary

Adds automated Docker image building workflow and improves GPG keyserver reliability.

Changes

1. New GitHub Actions Workflow (.github/workflows/docker_push.yml)

• Builds all 9 Docker images (Flink 1.20/2.0/2.1 with Java 8/11/17/21) on push to master and dev-* branches
• Pushes to GHCR (ghcr.io/{owner}/flink-docker) with multi-platform support (amd64, arm64)
• Automatically extracts version from Dockerfiles for tagging
• Enables testing on forks (adapts to repository owner)

2. Updated GPG Keyservers in All Dockerfiles

• Replaced deprecated ha.pool.sks-keyservers.net with modern hkps://keys.openpgp.org
• Affects both gosu and Flink tarball verification in all 9 Dockerfiles
• Fixes unreliable GPG key fetching that caused build failures

3. Reimplemented publish-to-dockerhub.sh

• Now uses crane to copy images from GHCR → Docker Hub (instead of local builds)
• 10x faster: ~10 minutes vs ~2 hours
• Platform independent, preserves multi-arch images automatically
• Adds interactive confirmation prompt before publishing
• Follows same pattern as flink-kubernetes-operator

Benefits

• Automated releases: Images automatically built and available in GHCR on merge
• Faster publishing: PMC members can copy to Docker Hub in minutes
• More reliable: Modern keyservers prevent GPG verification failures
• Better testing: Fork-friendly workflow enables testing before Apache repo

[gaborgsomogyi]

cc @mbalassi

[mbalassi]

This is fantastic. The portion of work that is missing is to update the Readme on explaining how a new version should be added/updated.

We can do that as a quick follow up.