MENDELU/feat(docker): make Solr heap tunable via SOLR_HEAP on 9.x (#1346)

DSpace 9.x launches Solr with `runuser -u solr -- solr-foreground` and no
`-m` flag, so the heap was pinned to the image default (512m) with no runtime
knob. Forward SOLR_HEAP through runuser so the solr user's environment carries
it into solr-foreground, which honours it natively.

Default is 4g when SOLR_HEAP is unset, aligning 9.x with the 7.x fleet and
source PR #1309. Deliberate default change on 9.x
(512m -> 4g). Set SOLR_HEAP in .env to override (e.g. SOLR_HEAP=1g).

New feature carved out of the 9.x N/A branches in
dataquest-dev/dspace-customers#746.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: jr-rk

docker/docker-compose-rest.yml

@@ -146,7 +146,7 @@ services:
       precreate-core suggestion /opt/solr/server/solr/configsets/suggestion
       cp -r /opt/solr/server/solr/configsets/suggestion/* suggestion
       chown -R solr:solr /var/solr
-      runuser -u solr -- solr-foreground
+      runuser -u solr -- env SOLR_HEAP="${SOLR_HEAP:-4g}" solr-foreground
 volumes:
   assetstore:
   pgdata: