build fix 9

docs/DOCKER.md

@@ -247,6 +247,22 @@ Use the generated files like this:
 
 Do not put the `.pub` file or an `authorized_keys` line into `VPS_SSH_KEY` or `VPS_SSH_KEY_B64`. Gitea needs the private key; the VPS needs the public key.
 
+Before saving the Gitea secret, verify the private key locally:
+
+```bash
+head -n 1 ./rentaldrivego_gitea_deploy
+wc -c ./rentaldrivego_gitea_deploy
+ssh-keygen -y -f ./rentaldrivego_gitea_deploy > ./rentaldrivego_gitea_deploy.pub
+```
+
+The first line must look like this:
+
+```text
+-----BEGIN OPENSSH PRIVATE KEY-----
+```
+
+The private key is usually hundreds of bytes. If the workflow reports a decoded byte count such as `79`, the Gitea secret is not the full private key. It is usually a public key, a fingerprint, or a truncated value.
+
 To install the public key on the VPS, SSH into the server as the same account configured in `VPS_USER`, then run:
 
 ```bash