add mas healthcheck

compose.mas.yml

@@ -21,6 +21,23 @@ services:
       - mas_encryption
       - mas_synapse_shared
       - mas_signing_rsa
+    # Official image is distroless (no curl/wget); upstream suggests `mas-cli config check` for probes.
+    # See https://github.com/element-hq/matrix-authentication-service/issues/3741 — validates config, not HTTP.
+    # GET /health is still served (resource `health` in mas.config.yaml.tmpl) for probes from other images.
+    healthcheck:
+      test:
+        [
+          "CMD",
+          "/usr/local/bin/mas-cli",
+          "--config",
+          "/etc/mas/config.yaml",
+          "config",
+          "check",
+        ]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+      start_period: 60s
     deploy:
       restart_policy:
         condition: on-failure