fix(docker): bun cache stays out of the runtime image, env= on a cache mount sets nothing

template/{{ part_dir }}/{% if serve != 'api' %}Dockerfile{% endif %}.jinja

@@ -25,12 +25,15 @@ RUN bun --bun run build
 FROM oven/bun:alpine AS runtime
 
 ENV NODE_ENV=production
+# The cache mount only holds files; without the variable bun caches into
+# ~/.bun inside the image.
+ENV BUN_INSTALL_CACHE_DIR=/bun-cache
 
 WORKDIR /app
 
 COPY package.json bun.lock* ./
 
-RUN --mount=type=cache,target=/bun-cache,env=BUN_INSTALL_CACHE_DIR bun install --frozen-lockfile --production
+RUN --mount=type=cache,target=/bun-cache bun install --frozen-lockfile --production
 
 COPY --from=builder /app/build ./build