fix: bake APP_VERSION into image as build arg instead of runtime env

- Dockerfile: ARG/ENV APP_VERSION, copies VERSION file
- compose: passes build arg, removes runtime env var
- webui: reads VERSION file as fallback if env not set
- deploy: explicit build step with --build-arg

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -16,9 +16,12 @@ ENV GIT_AUTHOR_NAME=outline-sync \
 RUN git config --global user.email "sync@local" && \
     git config --global user.name "outline-sync"
 
+ARG APP_VERSION=dev
+ENV APP_VERSION=${APP_VERSION}
+
 WORKDIR /work
 
-COPY outline_sync.py webui.py entrypoint.sh ./
+COPY outline_sync.py webui.py entrypoint.sh VERSION ./
 RUN chmod +x entrypoint.sh
 
 # Initialise vault with both branches needed by outline_sync.py