Fix default uid/gid of node_modules

backend/Dockerfile

@@ -1,9 +1,9 @@
 FROM alpine:3.12
 
-ENV UID=1000 GID=1000
-RUN export user=youtube \
-  && addgroup -S $user -g $GID && adduser -D -S $user -G $user -u $UID
-USER $user
+ENV UID=1000 \
+  GID=1000 \
+  USER=youtube
+RUN addgroup -S $USER -g $GID && adduser -D -S $USER -G $USER -u $UID
 
 RUN apk add --no-cache \
     ffmpeg \
@@ -13,6 +13,8 @@ RUN apk add --no-cache \
   && apk add --no-cache --repository http://dl-cdn.alpinelinux.org/alpine/edge/testing/ \
     atomicparsley
 
+# switch to default user to not chown node_modules on startup
+USER $USER
 WORKDIR /app
 
 COPY --chown=$UID:$GID [ "package.json", "package-lock.json", "/app/" ]
@@ -21,5 +23,7 @@ RUN npm install
 COPY --chown=$UID:$GID [ "./", "/app/" ]
 
 EXPOSE 17442
+# switch back to root to allow UID/GID changing
+USER root
 ENTRYPOINT [ "/app/entrypoint.sh" ]
 CMD [ "node", "app.js" ]