fix: bake version into Docker image at build time

- Add VERSION build arg to Dockerfile
- Pass --build-arg VERSION in release workflow
- Remove VERSION env override from docker-compose files
- Version is now immutable inside the image, no runtime env var needed

backend/Dockerfile

@@ -1,5 +1,9 @@
 FROM python:3.11-slim
 
+# Bake the version into the image at build time
+ARG VERSION=unknown
+ENV VERSION=${VERSION}
+
 # Security: run as non-root
 RUN groupadd -r aoc && useradd -r -g aoc aoc