Streamline Dockerfile, remove unused deps

* Dockerfile: Use non-root user, buildx cache, setup for readonly
   container, remove unused apt deps.
   For now don't install pip package and keep development flask server
   as this will be replaced in the future (fastapi). Then a proper
   webserver (e.g. nginx) should be used and the pip package can be
   created and deployed just to the run-stage (with the webserver).
 * docker-compose: Set to readonly (anonymous volumes declared in
   Dockerfile should maintain all writable data).
   Mount config.py for easier development. Should be replaced by
   environment support for all config file variables.
 * Remove unused runtime dependencies: mariadb, joblib, pytest,
   pytest-cov.
 * Move pytest-cov to dev dependencies.
 * Add output_dir to config.py.
 * Fix visualization_results.pdf endpoint.
 * Update docs.

tests/test_class_optimize.py

@@ -4,6 +4,7 @@ from pathlib import Path
 import pytest
 
 from akkudoktoreos.class_optimize import optimization_problem
+from akkudoktoreos.config import output_dir
 
 DIR_TESTDATA = Path(__file__).parent / "testdata"
 
@@ -35,5 +36,5 @@ def test_optimize(fn_in, fn_out):
     assert set(ergebnis) == set(expected_output_data)
 
     # The function creates a visualization result PDF as a side-effect.
-    fp_viz = Path(".") / "visualization_results.pdf"
+    fp_viz = Path(output_dir) / "visualization_results.pdf"
     assert fp_viz.exists()