gradle-docker-plugin: Do not parse config file if it doesn't exist

Hi,

First off, thanks for producing such a useful plugin, I use it a lot and think it is great!

I have recently updated one of my projects to use version 6.0.0 and I have everything working fine, but I am finding that an exception is thrown and logged which is creating a bit of noise in my logs, so I was wondering if there might be a way around it?

It seems to be coming from the way the plugin is trying to lookup auth config, I wondered if it might just be the setup on my local mac, but it is happening on my travis CI builds too (the project is private at the moment but I was planning on making it public soon, I can do that earlier if seeing the whole logs would help?)

The exception that is being thrown is:

Failure when attempting to lookup auth config (docker repository: , configFile: /home/travis/.docker/config.json. Falling back to docker-java default behaviour
groovy.json.JsonException: Unable to process file: /home/travis/.docker/config.json
	at org.apache.groovy.json.internal.BaseJsonParser.parse(BaseJsonParser.java:158)
	at groovy.json.JsonSlurper.parseFile(JsonSlurper.java:365)
	at groovy.json.JsonSlurper.parse(JsonSlurper.java:348)
	at com.bmuschko.gradle.docker.internal.RegistryAuthLocator.lookupAuthConfig(RegistryAuthLocator.groovy:109)
	at com.bmuschko.gradle.docker.internal.RegistryAuthLocator.lookupAuthConfig(RegistryAuthLocator.groovy:81)
	at com.bmuschko.gradle.docker.tasks.image.DockerPushImage.runRemoteCommand(DockerPushImage.groovy:55)
	at com.bmuschko.gradle.docker.tasks.AbstractDockerRemoteApiTask.start(AbstractDockerRemoteApiTask.groovy:73)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

When I run on my local mac it is similar but not exactly the same, although it does look to be the same sort of problem:

Failure when attempting to lookup auth config (docker repository: , configFile: /Users/michaelruocco/.docker/config.json. Falling back to docker-java default behaviour
groovy.json.JsonException: null not parse properly

The current character read is 'n' with an int value of 110
null not parse properly
line number 1
index number 1
no credentials server URL
.^
        at org.apache.groovy.json.internal.JsonParserCharArray.decodeNull(JsonParserCharArray.java:234)
        at org.apache.groovy.json.internal.JsonParserCharArray.decodeValueInternal(JsonParserCharArray.java:178)
        at org.apache.groovy.json.internal.JsonParserCharArray.decodeValue(JsonParserCharArray.java:157)
        at org.apache.groovy.json.internal.JsonParserCharArray.decodeFromChars(JsonParserCharArray.java:46)
        at org.apache.groovy.json.internal.JsonParserCharArray.parse(JsonParserCharArray.java:384)
        at org.apache.groovy.json.internal.BaseJsonParser.parse(BaseJsonParser.java:112)
        at groovy.json.JsonSlurper.parseText(JsonSlurper.java:205)
        at com.bmuschko.gradle.docker.internal.RegistryAuthLocator.runCredentialProvider(RegistryAuthLocator.groovy:252)
        at com.bmuschko.gradle.docker.internal.RegistryAuthLocator.authConfigUsingStore(RegistryAuthLocator.groovy:271)
        at com.bmuschko.gradle.docker.internal.RegistryAuthLocator.lookupAuthConfig(RegistryAuthLocator.groovy:123)
        at com.bmuschko.gradle.docker.internal.RegistryAuthLocator.lookupAuthConfig(RegistryAuthLocator.groovy:81)
        at com.bmuschko.gradle.docker.tasks.image.DockerPushImage.runRemoteCommand(DockerPushImage.groovy:55)

In either case the plugin says that it will fall back to the default behaviour and then my image is still pushed as required so there is no functional problem. I tried to look at the docs and config to see if there was a way to turn off the auth config lookup (with the idea it would make the plugin just use the default behaviour and bypass the auth config lookup, but I wasn’t able to find anything. Feel free to point me in the right direction if I have missed it!

For reference the relevant config that I have in my build.gradle file is:

plugins {
    id 'com.bmuschko.docker-remote-api' version '6.0.0'
}
...
import com.bmuschko.gradle.docker.tasks.image.DockerBuildImage;
import com.bmuschko.gradle.docker.tasks.image.DockerPushImage;
...
docker {
    registryCredentials {
        username = System.getenv("DOCKER_USERNAME")
        password = System.getenv("DOCKER_PASSWORD")
    }
}

task buildImage(type: DockerBuildImage) {
    inputDir.set(file('.'))
    images.add('michaelruocco/verification-context-spring-app:latest')
}

task pushImage(type: DockerPushImage) {
    images.set(buildImage.images)
}

As I said feel free to point me in the right direction if I am just doing something stupid and missing some config, but any input you can offer would be much appreciated!

Thanks.