docker: Use tini unilaterally and stop tailing /dev/null

tini is a hyper-minimal PID 0 which spawns a child process (watchfrr.sh
in our case), reaps zombies and forwards signals to the script. Starting
watchfrr.sh directly instead of through the old `tail /dev/null` or
`sleep 365d` helps keep things clean too :)

While tini was previously only used in the Alpine container it is useful
to apply this PID 0 to all containers except the special CI ones.

Fixes: #8788
Signed-off-by: Wesley Coakley <w@wesleycoakley.com>

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