perf(ci): optimize image sizes and caching to improve update and pipeline durations

The base image build has nothing mounted at UV_CACHE_DIR ($SHARED_DIR/.uv-cache), so uv sync wrote its ~423 MiB download cache into the image layer. At runtime and during site builds the shared folder is bind-mounted over that path, so the baked copy is hidden and never read.

Declare ARG UV_NO_CACHE=1 in the base image Containerfile rendered by juice build. An ARG is visible to RUN but not persisted in the image config, so the runtime UV_CACHE_DIR is unchanged. Site environment images are rendered through a different path and keep using their persistent, bind-mounted cache.

Related to #420

Co-Authored-By: Claude Opus 5.5 noreply@anthropic.com