Create /var/lib/pixa directory in Docker image for database/cache

- Change default StateDir from ./data to /var/lib/pixa (proper Unix convention)
- Create directory owned by pixad user in Dockerfile
- Set WORKDIR to /var/lib/pixa

Dockerfile

@@ -35,9 +35,13 @@ RUN apk add --no-cache \
 # Copy binary from builder
 COPY --from=builder /pixad /usr/local/bin/pixad
 
-# Create non-root user
-RUN adduser -D -H -s /sbin/nologin pixad
+# Create non-root user and data directory
+RUN adduser -D -H -s /sbin/nologin pixad && \
+    mkdir -p /var/lib/pixa && \
+    chown pixad:pixad /var/lib/pixa
+
 USER pixad
+WORKDIR /var/lib/pixa
 
 EXPOSE 8080