feat(mastodon): Make sidekiq run with readOnlyFilesystem: true

This patch introduces and ephemeral volume in order to allow sidekiq to
do the media processing without the problem that emptydir causes due to
how the ruby tempfile libarary evaluates temp file locations. Therefore
a small volume is introduced that takes care of it instead.

Additionally it refactors some of the volume templates to reduce
repetition.

References:
https://kubernetes.io/docs/concepts/storage/ephemeral-volumes/#generic-ephemeral-volumes
https://microblog.shivering-isles.com/@sheogorath/111040613569754423