Remove redundant Docker test build publishing

[wollomatic]

The Build and Publish Testing action currently:

• Pushes test builds to both Docker Hub and GHCR (redundant).
• Tags builds with both testing and testing-<sha> (problematic because it doesn't indicate which specific build it is).

Proposed changes:

• Remove Docker Hub pushes (keep GHCR only).
• Remove the testing tag (keep testing-<sha>).