merge #5097: update Dockerfile

- remove a layer and reduce image size
- update pip and apk

Dockerfile

@@ -1,5 +1,7 @@
 FROM python:alpine
-RUN python3 -m pip install -U gallery-dl yt-dlp
+RUN python3 -m pip install --no-cache-dir -U pip  && \
-RUN apk update
+    python3 -m pip install --no-cache-dir -U gallery-dl yt-dlp
-RUN apk add ffmpeg
+RUN apk update && \
+    apk add --no-cache ffmpeg && \
+    rm -rf /var/cache/apk/*
 ENTRYPOINT [ "gallery-dl" ]