Fix permission error in Dockerfile

- Create deployed_at.txt before switching to elixir user
- Set proper ownership on the file
- This fixes the 'Permission denied' error during build

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

Dockerfile

@@ -57,9 +57,12 @@ RUN apt-get update -y && \
 ENV LANG=en_US.UTF-8 LANGUAGE=en_US:en LC_ALL=en_US.UTF-8
 
 WORKDIR /app
+
+# Create deployment timestamp before switching users
+RUN date -u +"%Y-%m-%dT%H:%M:%SZ" > /app/deployed_at.txt && \
+    chown elixir:root /app/deployed_at.txt
+
 COPY --from=builder --chown=elixir:root /app/_build/prod/rel/aprsme ./
+
 USER elixir
-
-RUN date -u +"%Y-%m-%dT%H:%M:%SZ" > /app/deployed_at.txt
-
 CMD ["/app/bin/server"]