Development: accept PRUNE_PACKAGE_CACHE argument in Dockerfile

Use `ARG` in Dockerfile to invalidate the cache when this argument changes.

Requires readthedocs/common#244

Author: humitos

dockerfiles/Dockerfile

@@ -64,6 +64,7 @@ RUN ln -s /usr/bin/python3 /usr/bin/python
 
 WORKDIR /tmp
 
+ARG PRUNE_PACKAGE_CACHE=0
 COPY requirements/docker.txt docker.txt
 RUN ~/.local/bin/uv pip sync --system docker.txt