dockerfile: Give more stack space to /sbin/tini.

/sbin/tini ends up running the Sass compiler, which seems to need a
larger thread stack size than the musl default.  This patch incorporates
a fix from
https://raw.githubusercontent.com/jubel-han/dockerfiles/master/common/stack-fix.c,
as described in https://github.com/sass/node-sass/issues/2031.

docker_entrypoint.sh

@@ -11,4 +11,4 @@ echo "Updating permissions..."
 find /mastodon -path /mastodon/public/system -prune -o -not -user mastodon -not -group mastodon -print0 | xargs -0 chown -f mastodon:mastodon
 
 echo "Executing process..."
-exec su-exec mastodon:mastodon /sbin/tini -- "$@"
+LD_PRELOAD=/lib/stack-fix.so exec su-exec mastodon:mastodon /sbin/tini -- "$@"