Some distros provide it in /bin/sed and others in both /bin/sed
and /usr/bin/sed. Use the more available one.
Reported-by: Konstantin Pavlov <thresh@nginx.com>
Fixes: ac64ffde57 "Improved readability of <docker-entrypoint.sh>."
Signed-off-by: Alejandro Colomar <alx@nginx.com>