improve signal handling, alpine

[sbaier1]

tini allows proper signal propagation and using exec to replace the shell as the top process tree element we can ensure proper propagation of the signal that is received by the container.

this is the way most well-known and official docker images handle signal propagation for java applications.

switch to alpine proposed simply because it allows easier install of tini without having to deal with gpg verification intermittent errors

[PlusMinus0]

Thanks you for your pull request, looks interesting.