docker: fix files uploaded with the wrong mime type - fixes #6384

The Docker image had no /etc/mime.types, so MIME types came only from
Go's built-in table plus rclone's small extra list, and many
extensions uploaded as application/octet-stream.

This installs Alpine's mailcap package to fix the problem.

Dockerfile

@@ -42,6 +42,7 @@ RUN echo "**** Install Dependencies ****" && \
     apk add --no-cache \
         ca-certificates \
         fuse3 \
+        mailcap \
         tzdata && \
     echo "Enable user_allow_other in fuse" && \
     echo "user_allow_other" >> /etc/fuse.conf