fix: skip Docker login and push steps for fork PRs

[gounthar]

Summary

Fixes Docker workflow failures when running from fork PRs by skipping login and push steps that require the GHCR_TOKEN secret.

Problem

The workflow was failing with "Password required" error when running on fork PRs (like the ones from gounthar/quickstart-tutorials):

GHCR_TOKEN: 
IS_FORK: true
##[error]Password required

GitHub doesn't provide repository secrets to fork PRs for security reasons, but the workflow was attempting to login and push to GHCR regardless.

Root Cause

The workflow detects forks with the IS_FORK check but doesn't use this check to skip Docker login and image push operations:

- name: Check if PR is from a fork
  run: echo "IS_FORK=$(if [ '${{ github.event.pull_request.head.repo.full_name }}' != '${{ github.repository }}' ]; then echo 'true'; else echo 'false'; fi)" >> $GITHUB_ENV

Then later, login and build-push steps would attempt to run even when IS_FORK == 'true'.

Solution

Added && env.IS_FORK == 'false' to the condition of:

1. Login to GHCR step (1 step)
2. All 11 build-and-push steps for different Docker images

This ensures these steps only run when the PR is from the upstream repository, where the GHCR_TOKEN secret is available.

Impact

• ✅ Fork PRs can now run successfully without Docker login/push failures
• ✅ Upstream PRs continue to build and push images as before
• ✅ No change to scheduled runs or direct pushes to branches
• ✅ Maintains security by not attempting operations that require unavailable secrets

Testing

This fix will allow fork PRs (like those from @gounthar) to pass the Docker workflow checks.