Changes

Upgrade redmine to 5.1.11 (security fixes)

As the official docker image still has 5.1.10 we're temporarily using a
custom build of the redmine base container.

Change-Id: Id06331c1f169cd02340730d5499bcb8a1ad3ea22
Closes: OS#6915

bpftrace: Introduce ksys_write_delay.bt

This will record latency of write() calls during ttcn3-bts-test run,
where some write() call on stderr seems to be taking ~0.32 seconds,
stalling the timerfd (71 times) and making osmo-bts exit.

Related: OS#6794
Change-Id: Ia1d2036ac7c6c9e2818d07e8787d857b3b3e5881