Don't create cache with apk, use cdn network

backend/Dockerfile

@@ -1,8 +1,8 @@
 FROM alpine:3.11
 
 RUN \ 
-apk add --update npm python ffmpeg && \
-apk add --repository http://dl-3.alpinelinux.org/alpine/edge/testing/ \
+apk add --no-cache npm python ffmpeg && \
+apk add --no-cache --repository http://dl-cdn.alpinelinux.org/alpine/edge/testing/ \
     atomicparsley
 
 # Change directory so that our commands run inside this new directory