Restore backend image release publishing

[pabo99]

Summary

Restore release workflow publishing for the backend Docker image.

This makes future releases publish:

• omegaup/backend:${VERSION}
• omegaup/backend:sha-${REVISION}
• ghcr.io/omegaup/backend:${VERSION}
• ghcr.io/omegaup/backend:sha-${REVISION}

It also updates docker/Dockerfile.backend from ubuntu:focal to ubuntu:22.04 and OpenJDK 17, so the image can run backend binaries built by the current Ubuntu 22.04 release workflow.

Why

v1.9.73 successfully published the GitHub release asset omegaup-backend.tar.xz, but did not publish docker.io/omegaup/backend:v1.9.73.

Production consumes omegaup/backend:${VERSION}, so the prod bump hit ImagePullBackOff.

A manual backend image build also exposed that the old ubuntu:focal base image cannot run the current release binaries because they require newer GLIBC versions.

Validation

• Parsed .github/workflows/release.yml as YAML.
• Ran git diff --check.
• Built locally:

    docker build --pull \
      --build-arg RELEASE=v1.9.73 \
      -f docker/Dockerfile.backend \
      -t quark-backend-test:v1.9.73 \
      docker

• Verified the local image:

    docker run --rm --platform linux/amd64 quark-backend-test:v1.9.73 /usr/bin/omegaup-grader -version
    docker run --rm --platform linux/amd64 quark-backend-test:v1.9.73 /usr/bin/omegaup-broadcaster -version
    docker image inspect quark-backend-test:v1.9.73 --format '{{.Architecture}}'

Results:

• omegaup-grader v1.9.73
• omegaup-broadcaster v1.9.73
• amd64

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 41.29%. Comparing base (97c3a2d) to head (4215b1c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #147   +/-   ##
=======================================
  Coverage   41.29%   41.29%           
=======================================
  Files          25       25           
  Lines        6868     6868           
=======================================
  Hits         2836     2836           
  Misses       3770     3770           
  Partials      262      262           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.