Make Solr heap tunable via SOLR_HEAP (default 4g, no behavior change)

Backport of #1309 to customer/lindat.
Translate (not cherry-pick) the single Solr heap line against this
branch's actual compose file: it launches Solr with -p 8983, so only the
-m 4g argument is changed to -m "${SOLR_HEAP:-4g}". With SOLR_HEAP
unset the 4g default is preserved; AlwaysPreTouch no longer forces the
full 4g commit when a smaller heap is set.

Refs dataquest-dev/dspace-customers#746

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: jr-rk

docker/docker-compose-rest.yml

@@ -175,7 +175,7 @@ services:
       cp -r /opt/solr/server/solr/configsets/search/* search
       precreate-core statistics /opt/solr/server/solr/configsets/statistics
       cp -r /opt/solr/server/solr/configsets/statistics/* statistics
-      exec solr -p 8983 -f -m 4g
+      exec solr -p 8983 -f -m "${SOLR_HEAP:-4g}"
 volumes:
   # Commented out because there are a lot of files in the assetstore
   assetstore: