Ensure the Docker nginx user can write servers.json. Fixes #1046

Dockerfile

@@ -9,7 +9,13 @@ ARG UID=101
 LABEL maintainer="Alejandro Celaya <[email protected]>"
 USER root
 RUN rm -r /usr/share/nginx/html && rm /etc/nginx/conf.d/default.conf
-USER $UID
 COPY config/docker/nginx.conf /etc/nginx/conf.d/default.conf
 COPY scripts/docker/servers_from_env.sh /docker-entrypoint.d/30-shlink-servers-json.sh
 COPY --from=node /shlink-web-client/build /usr/share/nginx/html
+
+# Allow the nginx user to create new files in the html dir, but not modify existing files.
+# This is required by 30-shlink-servers-json.sh
+RUN chown :$UID /usr/share/nginx/html && chmod g+wx /usr/share/nginx/html
+
+# Switch to non-privileged UID as the last step
+USER $UID