docker: add mailcap for /etc/mime.types - fixes #6384 - #9977

What is the purpose of this change?

The Docker image has no /etc/mime.types, so MIME types come only from Go's
built-in table plus rclone's small extra list, and many extensions upload as
application/octet-stream. This installs Alpine's mailcap package, as
suggested in the issue.

Checked by running rclone lsf --format pm over one file per extension
listed in mime.types (1393 extensions):

• octet-stream results drop from 1313 to 9 (e.g. .tar, .rar, .heic, .jxl,
  .m4v, .flv, .aac, .woff2, .deb now get real types)
• image size grows by about 74 KiB (93.63 MB -> 93.70 MB)

As on any Linux host with mime.types, the system file takes precedence, so a
few types change: .ts becomes text/vnd.trolltech.linguist (was video/MP2T),
.flac audio/x-flac, .wav audio/x-wav. If keeping video/MP2T for .ts matters,
I can add an override to fs/mimetype.go.

Was the change discussed in an issue or in the forum before?

Fixes #6384

Checklist

• I have read the contribution guidelines.
• I have added tests for all changes in this PR if appropriate.
• I have added documentation for the changes if appropriate.
• All commit messages are in house style.
• I'm done, this Pull Request is ready for review :-)