[docker] update Dockerfile

- add LANG=C.UTF-8
  (better utf-8 support or so I heard)
- restructure RUN commands to be more easily extendable and have
  simpler git diffs
- swap 'apk' and 'pip' since 'apk' results are more likely to be stable
  and therefore cacheable
- disable Python bytecode caching with -B
  - reduces image size by 10Mb
  - bytecode for explicitly installed packages (gallery-dl, yt-dlp,
    dependencies) is still cached
- add some (hopefully) helpful command-line arguments to pip

Dockerfile

@@ -1,7 +1,19 @@
 FROM python:alpine
-RUN python3 -m pip install --no-cache-dir -U pip  && \
-    python3 -m pip install --no-cache-dir -U gallery-dl yt-dlp
-RUN apk update && \
-    apk add --no-cache ffmpeg && \
-    rm -rf /var/cache/apk/*
+ENV LANG=C.UTF-8
+
+RUN : \
+    && apk --no-interactive update \
+    && apk --no-cache --no-interactive add ffmpeg \
+    && rm -rf /var/cache/apk \
+    && :
+
+RUN : \
+    && python3 -B -m pip --no-cache-dir --no-input --disable-pip-version-check install -U \
+        pip \
+    && python3 -B -m pip --no-cache-dir --no-input --disable-pip-version-check install -U \
+        gallery-dl \
+        yt-dlp \
+    && rm -rf /root/.cache/pip \
+    && :
+
 ENTRYPOINT [ "gallery-dl" ]