Docs feedback: recommended tetraweb/php is broken

Affected docs pages:

• Running Composer and NPM scripts with deployment via SCP in GitLab CI/CD | GitLab
• Probably others too

The mentioned docker image tetraweb/php currently is broken ([Critical] debian repositories broken · Issue #64 · TetraWeb/docker, Forum: Gitlab runners stopped working - Questions & Answers / GitLab CI - GitLab Forum).

We should fix this or settle on a robust replacement.