docker: switch Ruby memory allocator to Jemalloc.

Switch the Ruby memory allocator from Glibc malloc to Jemalloc. Jemalloc
supposedly uses less memory than Glibc malloc because it's better at
handling memory fragmentation. It also has detailed internal statistics
to help monitor allocator behavior.

We use the LD_PRELOAD method of loading Jemalloc instead of building it
into Ruby so that we can switch allocators at runtime.

config/docker/Dockerfile.danbooru

@@ -3,10 +3,6 @@ FROM ubuntu:21.04 AS base
 WORKDIR /danbooru
 ENV PATH="/root/.asdf/bin:/root/.asdf/shims:$PATH:/usr/lib/postgresql/14/bin"
 
-# Reduces memory usage at the cost of higher thread contention.
-# https://bugs.ruby-lang.org/issues/14718
-ENV MALLOC_ARENA_MAX=2
-
 COPY config/docker/build-base-image.sh .
 
 RUN \
@@ -64,6 +60,8 @@ RUN \
 
 FROM base as production
 
+ENV LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libjemalloc.so.2
+
 COPY --from=development /root /root
 COPY --from=development /danbooru /danbooru