Plugin Doesn't Respect ~/.docker/config.json "credHelpers" settings

We are currently using the Docker plugin to run builds Docker images on a remote server.

Previously, we have been hosting our Docker images on a private Artifactory repository.

Recently, we moved our images to a private Google Container Repository (GCR) repo.

Our Jenkins master and Docker hosts are running on GCE, so they have been set up with the `credHelper` settings in `/home/jenkins/.docker/config.json`:

```
$ cat /home/jenkins/docker/config.json
{
"credHelpers": {
"gcr.io": "gcloud",
"us.gcr.io": "gcloud",
"eu.gcr.io": "gcloud",
"asia.gcr.io": "gcloud",
"staging-k8s.gcr.io": "gcloud",
"marketplace.gcr.io": "gcloud"
}
}
```

This allows the Jenkins user to pull images via the CLI, and authentication is handled via the underlying `gcloud` library, using the GCE instance's machine account.

However, the Jenkins Docker plugin requires a Service Account JSON key and cannot - it seems - use the GCE machine account, either via a Jenkins Credential or by specifying `none` and allowing the Java library to fall back to the native configuration. In each case, the below exception is shown on the Jenkins master's "Configure Clouds" page:

```
com.github.dockerjava.api.exception.InternalServerErrorException: {"message":"unauthorized: You don't have the needed permissions to perform this operation, and you may have invalid credentials. To authenticate your request, follow the steps in: https://cloud.google.com/container-registry/docs/advanced-authentication"}
at com.github.dockerjava.netty.handler.HttpResponseHandler.channelRead0(HttpResponseHandler.java:109)
at com.github.dockerjava.netty.handler.HttpResponseHandler.channelRead0(HttpResponseHandler.java:33)
at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:105)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at io.netty.handler.logging.LoggingHandler.channelRead(LoggingHandler.java:241)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at io.netty.channel.CombinedChannelDuplexHandler$DelegatingChannelHandlerContext.fireChannelRead(CombinedChannelDuplexHandler.java:438)
at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:310)
at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:284)
at io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:253)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at io.netty.handler.timeout.IdleStateHandler.channelRead(IdleStateHandler.java:287)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1334)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:926)
at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:134)
at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:644)
at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:579)
at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:496)
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:458)
at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:858)
at io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:138)
at java.lang.Thread.run(Thread.java:748)

```

This has been tested and verified by changing the image to a publicly-accessible GCR image - the exception does not occur here because the unauthenticated client can access it.

- docker-plugin version -` 1.1.8`
- jenkins version you use - `2.222.4`
- docker engine version you use - `19.03.8, build afacb8b`