try to fix ffmpeg install from edge

Dockerfile

@@ -25,15 +25,16 @@ ENV NO_UPDATE_NOTIFIER=true
 
 RUN addgroup -S $USER -g $GID && adduser -D -S $USER -G $USER -u $UID
 
+RUN apk -UvX http://dl-cdn.alpinelinux.org/alpine/edge/community/ add -u\
+    ffmpeg
+
 RUN apk add --no-cache \
   npm \
   python2 \
   python3 \
   su-exec \
   && apk add --no-cache --repository http://dl-cdn.alpinelinux.org/alpine/edge/testing/ \
-    atomicparsley \
-  && apk add --no-cache --repository http://dl-cdn.alpinelinux.org/alpine/edge/community/ \
-    ffmpeg
+    atomicparsley
 
 WORKDIR /app
 COPY --chown=$UID:$GID [ "backend/package.json", "backend/package-lock.json", "/app/" ]