Reduce IUK size

This is about researching low-hanging fruits to reduce the size of our IUKs in order to be nicer to users with not very fast Internet connection. Regarding increased memory consumption caused by the “single SquashFS diff” idea, another approach (#6876) will solve the problem.

Current best ideas (collected initially on #11211 (closed) and #11345 (closed); see also #6425 (closed)) are:

• apply the 99-set_mtimes trick to:
  • /usr/share/mime/application/*
  • /var/cache/debconf/*
  • /var/lib/dpkg/info
• delete /var/lib/dkms/ during the build, then remove the dkms bits from config/chroot_local-hooks/99-zzzzzz_reproducible-builds-post-processing

To evaluate the impact of these ideas:

1. Fork the tag for Tails version N, apply the aforementioned tricks. Build an ISO.
2. Do the same for Tails version N+1.
3. Build the IUK from N to N+1 using these ISOs.
4. Compare the size of the IUK you’ve built with the size of the IUK we’ve published from N to N+1.

Related issues

• Related to #15281
• Related to #15289
• Related to #6425 (closed)
• Has duplicate #11211 (closed)
• Has duplicate #11345 (closed)

Original created by @anonym on 15290 (Redmine)