Initial Dockge image update checker

SECURITY.md

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+# Security
+
+Report security issues privately to the repository owner.
+
+## Runtime Access
+
+This service reads Dockge stack files and Docker Engine image metadata. Mounting the Docker socket gives broad host-level power to any process with write access to it, so deploy this service only in trusted environments and keep the socket mount read-only at the container level.
+
+## Secrets
+
+Do not commit registry credentials, Gitea tokens, `.env` files, private keys, certificates, or Docker config files.