fix(deploy) : add minimal .env file in Docker image for Symfony boot

deploy/docker/Dockerfile.prod

@@ -67,6 +67,9 @@ COPY --from=backend-build /app /var/www/html
 # Frontend from stage 2
 COPY --from=frontend-build /app/frontend/.output/public /var/www/html/frontend/.output/public
 
+# Symfony needs a .env file to boot (variables are overridden by env_file in docker-compose)
+RUN echo "APP_ENV=prod" > /var/www/html/.env
+
 # Permissions
 RUN mkdir -p /var/www/html/var \
     && chown -R www-data:www-data /var/www/html/var