docker: set MALLOC_CONF in docker image.

Set the MALLOC_CONF environment variable in the Docker image to tune the
Jemalloc configuration. Configuring Jemalloc to use two memory arenas
reduces memory fragmentation, and using background threads and low decay
times allows freed memory to be returned to the OS sooner.

Previously we set this environment variable at runtime in Kubernetes,
but baking it into the image is simpler.

config/docker/Dockerfile.danbooru

@@ -61,6 +61,7 @@ RUN \
 FROM base as production
 
 ENV LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libjemalloc.so.2
+ENV MALLOC_CONF=background_thread:true,narenas:2,dirty_decay_ms:1000,muzzy_decay_ms:0,tcache:false
 
 COPY --from=development /root /root
 COPY --from=development /danbooru /danbooru