Workaround for running docker-pihole (w/ s6-overlay v3) on fly.io

Dockerfile

FROM pihole/pihole:latest

# Run s6 in its own PID namespace so that it is PID 1, even when the
# environment already has its own init process (fly.io has a non-optional
# one), to avoid this error:
#     s6-overlay-suexec: fatal: can only run as pid 1
#
# Un-ignore SIGINT and SIGTERM signals so that s6-init and all its child
# processes don't default to ignoring those signals.  A bug in the "unshare"
# utility propagates ignoring these signals to the child process, which is
# then inherited to the other children. This causes "service pihole-FTL
# restart" to hang, because the bash "run" script ignores the SIGTERM.
#
# Fixed in util-linux 2.38 and above
# (https://github.com/util-linux/util-linux/pull/1560).
#
# You don't want to do this unless there's already an init, because if
# "unshare" is the actual PID 1:
#     % docker exec -it pihole ls
#     OCI runtime exec failed: exec failed: unable to start container process: error closing exec fds: open /proc/self/fd: no such file or directory:  unknown
#

ENTRYPOINT [ \
    "unshare", "--pid", "--fork", "--kill-child=SIGTERM", "--mount-proc", \
    "perl", "-e", "$SIG{INT}=''; $SIG{TERM}=''; exec @ARGV;", "--", \
    "/s6-init" ]