add additional note on gcloud credentials

docs/private-registries.md

@@ -1,12 +1,17 @@
-Watchtower supports private Docker image registries. In many cases, accessing a private registry requires a valid username and password (i.e., _credentials_). In order to operate in such an environment, watchtower needs to know the credentials to access the registry. 
+Watchtower supports private Docker image registries. In many cases, accessing a private registry
+requires a valid username and password (i.e., _credentials_). In order to operate in such an
+environment, watchtower needs to know the credentials to access the registry. 
 
-The credentials can be provided to watchtower in a configuration file called `config.json`. There are two ways to generate this configuration file:
+The credentials can be provided to watchtower in a configuration file called `config.json`.
+There are two ways to generate this configuration file:
 
 * The configuration file can be created manually.
 * Call `docker login <REGISTRY_NAME>` and share the resulting configuration file.
 
 ### Create the configuration file manually
-Create a new configuration file with the following syntax and a base64 encoded username and password `auth` string:
+Create a new configuration file with the following syntax and a base64 encoded username and
+password `auth` string:
+
 ```json
 {
     "auths": {
@@ -17,27 +22,40 @@ Create a new configuration file with the following syntax and a base64 encoded u
 }
 ```
 
-`<REGISTRY_NAME>` needs to be replaced by the name of your private registry (e.g., `my-private-registry.example.org`)
+`<REGISTRY_NAME>` needs to be replaced by the name of your private registry
+(e.g., `my-private-registry.example.org`)
 
 The required `auth` string can be generated as follows:
 ```bash
 echo -n 'username:password' | base64
 ```
 
-When the watchtower Docker container is started, the created configuration file (`<PATH>/config.json` in this example) needs to be passed to the container:
+> ### ℹ️ Username and Password for GCloud
+>
+> For gcloud, we'll use `__json_key` as our username and the content
+> of `gcloudauth.json` as the password.
+
+When the watchtower Docker container is started, the created configuration file
+(`<PATH>/config.json` in this example) needs to be passed to the container:
+
 ```bash
 docker run [...] -v <PATH>/config.json:/config.json containrrr/watchtower
 ```
 
 ### Share the Docker configuration file
-To pull an image from a private registry, `docker login` needs to be called first, to get access to the registry. The provided credentials are stored in a configuration file called `<PATH_TO_HOME_DIR>/.docker/config.json`. This configuration file can be directly used by watchtower. In this case, the creation of an additional configuration file is not necessary.
+To pull an image from a private registry, `docker login` needs to be called first, to get access
+to the registry. The provided credentials are stored in a configuration file called `<PATH_TO_HOME_DIR>/.docker/config.json`.
+This configuration file can be directly used by watchtower. In this case, the creation of an
+additional configuration file is not necessary.
 
 When the Docker container is started, pass the configuration file to watchtower:
+
 ```bash
 docker run [...] -v <PATH_TO_HOME_DIR>/.docker/config.json:/config.json containrrr/watchtower
 ```
 
 When creating the watchtower container via docker-compose, use the following lines:
+
 ```yaml
 version: "3"
 [...]