Update auth FAQ to add $DOCKER_CONFIG
@@ -526,6 +526,7 @@ If the registry returns `401 Unauthorized` or `"code":"UNAUTHORIZED"`, it is oft
* You did not configure auth information in the default places where Jib searches.
- `$HOME/.docker/config.json`, [one of the configuration files](https://docs.docker.com/engine/reference/commandline/cli/#configuration-files) for the `docker` command line tool. See [configuration files document](https://docs.docker.com/engine/reference/commandline/cli/#configuration-files), [credential store](https://docs.docker.com/engine/reference/commandline/login/#credentials-store) and [credential helper](https://docs.docker.com/engine/reference/commandline/login/#credential-helpers) sections, and [this](https://github.com/GoogleContainerTools/jib/issues/101) for how to configure auth. For example, you can do `docker login` to save auth in `config.json`, but it is often recommended to configure a credential helper (also configurable in `config.json`).
+ - (Starting from Jib 2.2.0) You can set the environment variable `$DOCKER_CONFIG` for the _directory_ containing Docker configuration files. `$DOCKER_CONFIG/config.json` takes precedence over `$HOME/.docker/config.json`.
- Some common credential helpers on `$PATH` (for example, `docker-credential-osxkeychain`, `docker-credential-ecr-login`, etc.) for well-known registries.
- Jib configurations
- Configuring credential helpers: [`<from/to><credHelper>`](https://github.com/GoogleContainerTools/jib/tree/master/jib-maven-plugin#using-docker-credential-helpers) (Maven) / [`from/to.credHelper`](https://github.com/GoogleContainerTools/jib/tree/master/jib-gradle-plugin#using-docker-credential-helpers) (Gradle)
