feat: allow override startup command for Docker image (#3800)

This is to enable the use case where the stock Docker image is used with
different flags. E.g. `docker run xhofe/alist:latest ./alist server --data=mydata`

This was the behavior until PR#2818 changed it. This would make the image more usable.

Dockerfile

@@ -15,4 +15,4 @@ RUN apk add --no-cache bash ca-certificates su-exec tzdata; \
     chmod +x /entrypoint.sh
 ENV PUID=0 PGID=0 UMASK=022
 EXPOSE 5244
-ENTRYPOINT [ "/entrypoint.sh" ]
+CMD [ "/entrypoint.sh" ]