Use prebuilt Docker image for Gitea workflows

runner-image/Dockerfile

@@ -0,0 +1,26 @@
+FROM node:20-bookworm
+
+RUN apt-get update && \
+    DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
+      ca-certificates \
+      git \
+      python3 \
+      python3-pip \
+      python3-venv \
+      tzdata && \
+    rm -rf /var/lib/apt/lists/*
+
+COPY requirements.lock /tmp/requirements.lock
+
+RUN python3 -m venv /opt/blackout-venv && \
+    /opt/blackout-venv/bin/pip install --no-cache-dir -r /tmp/requirements.lock && \
+    /opt/blackout-venv/bin/python -c "import dotenv, jdatetime, requests" && \
+    /opt/blackout-venv/bin/pytest --version && \
+    /opt/blackout-venv/bin/ruff --version && \
+    rm /tmp/requirements.lock
+
+ENV PATH="/opt/blackout-venv/bin:$PATH" \
+    PYTHONDONTWRITEBYTECODE="1" \
+    PYTHONUNBUFFERED="1"
+
+CMD ["bash"]