feat: add shared runner Dockerfile and document rebuild process

All wiki pipelines share novoyuuparosk-wiki-runner:latest — a
locally-built image with git, pandoc, ca-certificates, and Python
packages pre-installed. Eliminates the apt-get and pip install
steps from every job run. Rebuild manually on the Pi when deps change.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,8 @@
+FROM python:3.12-slim
+
+RUN apt-get update -qq \
+    && apt-get install -y --no-install-recommends git pandoc ca-certificates \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY pipelines/songs/requirements.txt /tmp/songs-requirements.txt
+RUN pip install --no-cache-dir -r /tmp/songs-requirements.txt