Make MySQL image configurable for CPUs without x86-64-v2

mysql:8.4 (Oracle Linux 9 build) aborts with 'Fatal glibc error: CPU does
not support x86-64-v2' on generic VM CPU types (kvm64/qemu64) and pre-2009
hardware. MYSQL_IMAGE env var (default mysql:8.4) lets such hosts run the
baseline-x86-64 mysql:8.0-debian build instead.

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

docker-compose.yml

@@ -2,7 +2,10 @@ name: pesco-ncr
 
 services:
   mysql:
-    image: mysql:8.4
+    # mysql:8.4 (Oracle Linux 9) requires an x86-64-v2 CPU. On hosts/VMs
+    # without it ("Fatal glibc error: CPU does not support x86-64-v2"),
+    # set MYSQL_IMAGE=mysql:8.0-debian in .env (baseline x86-64 build).
+    image: ${MYSQL_IMAGE:-mysql:8.4}
     command:
       - --character-set-server=utf8mb4
       - --collation-server=utf8mb4_unicode_ci