@Override
    @Nonnull
    public String apply(@Nonnull final RegistryAuthSupplier input) {
      return input.getClass().getSimpleName();
    }
Attempt to load a GCR compatible RegistryAuthSupplier based on a few conditions:
<ol>
<li>First check to see if the environemnt variable DOCKER_GOOGLE_CREDENTIALS is set and points
to a readable file</li>
<li>Otherwise check if the Google Application Default Credentials can be loaded</li>
</ol>
Note that we use a special environment variable of our own in addition to any environment
variable that the ADC loading uses (GOOGLE_APPLICATION_CREDENTIALS) in case there is a need for
the user to use the latter env var for some other purpose in their build.

@return a GCR RegistryAuthSupplier, or null
@throws MojoExecutionException if an IOException occurs while loading the explicitly-requested
                               credentials
src/main/java/com/spotify/docker/AbstractDockerMojo.java