Drop grep from the Solr probe by asking the core's ping handler

Follow-up to review feedback that `grep` is as implicit a dependency as `curl` was.

It is not, strictly - in Debian the two differ:

  grep: Essential=yes   (apt refuses to remove it; it is part of the base system)
  curl: Essential=no    (genuinely optional, which is why it is now installed explicitly)

But the objection points at something better than installing another package: the probe
does not need grep at all. Asking the core's own ping handler is enough, because an
unregistered core is not a route:

  curl -fsS -o /dev/null http://localhost:8983/solr/search/admin/ping

  registered core   -> HTTP 200, exit 0
  unloaded core     -> HTTP 404, exit 22

Verified by unloading and recreating the core against the running container.

This is a better probe than the CoreAdmin STATUS + grep it replaces:
  - depends on curl alone, no pipe and no second binary
  - does not match on Solr's JSON, which a Solr upgrade could reformat
  - ping executes a real query against the core, so it checks the core answers rather
    than merely appearing in a list

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: MatusBeke

docker-compose.yml

@@ -250,12 +250,17 @@ services:
       precreate-core statistics /opt/solr/server/solr/configsets/statistics
       cp -r /opt/solr/server/solr/configsets/statistics/* statistics
       exec solr -f
-    # A port check is not enough: precreate-core runs AFTER Solr starts listening,
-    # so a port probe goes green while DSpace would still fail every query.
-    # Asking for a specific core discriminates - a missing core returns
-    # {"status":{"search":{}}} with no "name", so the grep fails.
+    # Asks the core's own ping handler rather than the CoreAdmin list. A registered core
+    # answers 200; an unregistered one is simply not a route, so Solr answers 404 and
+    # curl -f fails. That is worth more than it looks:
+    #   - no pipe and no grep, so the probe depends on nothing but curl
+    #   - no matching on Solr's JSON, which could be reformatted by a Solr upgrade
+    #   - ping runs a real query against the core, so it checks the core actually
+    #     answers rather than merely appearing in a list
+    # A port check would be useless here: precreate-core runs AFTER Solr starts
+    # listening, so the port is up while every DSpace query would still fail.
     healthcheck:
-      test: ["CMD-SHELL", "curl -fsS 'http://localhost:8983/solr/admin/cores?action=STATUS&core=search' | grep -q '\"name\":\"search\"'"]
+      test: ["CMD-SHELL", "curl -fsS -o /dev/null http://localhost:8983/solr/search/admin/ping"]
       interval: 10s
       timeout: 5s
       retries: 5