UK/Make Solr heap tunable via SOLR_HEAP (default 4g, no behavior change) (#1329)

Backport of #1309 to customer/uk.
Translate (not cherry-pick) the single Solr heap line against this
branch's actual compose file: it launches Solr with -p 898${INSTANCE}, so only the
-m 4g argument is changed to -m "${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

@@ -150,7 +150,7 @@ services:
       cp -r -u /opt/solr/server/solr/configsets/dspace/search/* search
       precreate-core statistics /opt/solr/server/solr/configsets/dspace/statistics
       cp -r -u /opt/solr/server/solr/configsets/dspace/statistics/* statistics
-      exec solr -p 898${INSTANCE} -f -m 4g
+      exec solr -p 898${INSTANCE} -f -m "${SOLR_HEAP:-4g}"
 volumes:
   assetstore:
   pgdata: