build: Use PlantUML from Docker image to work with Renovate, remove pip cache - #92

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v2, docker/build-push-action@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Unexpected input(s) 'path', valid inputs are ['add-hosts', 'allow', 'attests', 'build-args', 'build-contexts', 'builder', 'cache-from', 'cache-to', 'cgroup-parent', 'context', 'file', 'labels', 'load', 'network', 'no-cache', 'no-cache-filters', 'outputs', 'platforms', 'provenance', 'pull', 'push', 'sbom', 'secrets', 'secret-files', 'shm-size', 'ssh', 'tags', 'target', 'ulimit', 'github-token']