Reasons for creating this PR

Fix the Dockerfile that broke in Skosmos 2.18. It broke because newer EasyRdf requires PHP 8.0+ while the old Dockerfile was based on Ubuntu 20.04 that has PHP 7.4.

Based on PR #1498 which did essentially the same for the Skosmos 3 Dockerfile. (But this PR doesn't remove gettext because Skosmos 2 needs it.)

Link to relevant issue(s), if any

• Closes Dockerfiles don't work on Skosmos 2.18 #1678

Description of the changes in this PR

• upgrade the Dockerfile to Ubuntu 22.04 base image

Known problems or uncertainties in this PR

n/a

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that show that the new code works, or tests are not relevant for this PR (e.g. only HTML/CSS changes)
• The PR doesn't reduce accessibility of the front-end code (e.g. tab focus, scaling to different resolutions, use of .sr-only class, color contrast)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)