fix(action): build payload with jq and document that swarm mode deploys the tag

Interpolating inputs straight into the JSON string let a quote in image or
tag break the payload. Also replaces GNU-only head -n -1 so the action runs
on BSD/macOS runners.

gitea-action/README.md

@@ -28,7 +28,7 @@ jobs:
 |---|---|---|---|
 | `endpoint` | yes | — | Full URL to `/update` |
 | `image` | yes | — | Image reference without tag |
-| `tag` | no | `""` | Tag that was just pushed (logged for audit) |
+| `tag` | no | `""` | Tag that was just pushed. Compose mode ignores it (the compose file pins the reference); **swarm mode sets the service image to it**, so omitting it deploys `:latest`. Always pass it. |
 | `token` | yes | — | Bearer token configured in package-updater |
 
 ## Failure modes