test: add unit tests and CI workflow for dockreap

- Add unittest-based unit tests for core volume logic
- Add Makefile with isolated venv-based test runner
- Add GitHub Actions CI workflow
- Automatically mark SemVer-tagged commits as stable

https://chatgpt.com/share/695107c4-f320-800f-b4ce-da953de9bb86

Makefile

@@ -0,0 +1,21 @@
+.ONESHELL:
+SHELL := /bin/bash
+.SHELLFLAGS := -euo pipefail -c
+
+VENV ?= .venv
+PY   := $(VENV)/bin/python
+PIP  := $(VENV)/bin/pip
+
+.PHONY: venv test clean
+
+venv:
+	test -x "$(PY)" || python3 -m venv "$(VENV)"
+	"$(PIP)" install -U pip
+	"$(PIP)" install -e .
+
+test: venv
+	"$(PY)" -m unittest discover -s tests/unit -p "test_*.py" -v
+
+clean:
+	rm -rf "$(VENV)" .pytest_cache .coverage
+	find . -type d -name "__pycache__" -print0 | xargs -0r rm -rf