Pre-create target dirs with storkit ownership in Dockerfile

Docker named volumes inherit directory ownership when first created.
By creating /workspace/target and /app/target as storkit-owned before
the USER directive, the volumes will be writable by the storkit user.
Without this, cargo build/test/clippy all fail with Permission Denied.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/Dockerfile

@@ -107,7 +107,9 @@ RUN groupadd -r storkit \
     && mkdir -p /home/storkit/.claude \
     && chown -R storkit:storkit /home/storkit \
     && chown -R storkit:storkit /usr/local/cargo /usr/local/rustup \
-    && chown -R storkit:storkit /app
+    && chown -R storkit:storkit /app \
+    && mkdir -p /workspace/target /app/target \
+    && chown storkit:storkit /workspace/target /app/target
 
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 # Validates required env vars (GIT_USER_NAME, GIT_USER_EMAIL) and