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.

Dockerfile

@@ -61,6 +61,9 @@ RUN apk -U upgrade \
  && rm -rf /tmp/* /var/cache/apk/*
 
 COPY Gemfile Gemfile.lock package.json yarn.lock .yarnclean /mastodon/
+COPY stack-fix.c /lib
+RUN gcc -shared -fPIC /lib/stack-fix.c -o /lib/stack-fix.so
+RUN rm /lib/stack-fix.c
 
 RUN bundle config build.nokogiri --with-iconv-lib=/usr/local/lib --with-iconv-include=/usr/local/include \
  && bundle install -j$(getconf _NPROCESSORS_ONLN) --deployment --without test development \