Remove the documentation section referencing gcr.io in favor of local build because we will stop publishing containers in the future (#59)

* First pass draft. Replace all references of "gcr.io" with a custom artifact registry string. Will review and adjust in subsequent commits.

* Formatted as an environmental variable

* Addressed feedback

Author: nrgoogle

README.md

@@ -43,21 +43,12 @@ cd marketplace-vm-imagebuilder
 
 ### Use the Docker image
 
-You can build the Docker image, or pull an image from the Google Container Registry (GCR) repository.
-
-If you want to build the `imagebuilder` container image, run the following `docker` command:
+To build the `imagebuilder` container image, run the following `docker` command:
 
 ```shell
 docker build -t imagebuilder .
 ```
 
-If you want to pull the `imagebuilder` container image, run the following `docker` command:
-
-```shell
-docker pull gcr.io/cloud-marketplace-tools/vm/imagebuilder:0.1.7
-docker tag gcr.io/cloud-marketplace-tools/vm/imagebuilder:0.1.7 imagebuilder
-```
-
 ### Set the environment variables for your build environment
 
 ```shell
@@ -96,7 +87,7 @@ The service account key might not be available when executing locally for manual
 gcloud auth application-default login
 ```
 
-Follow instructions to get authentication token and complete the login. 
+Follow instructions to get authentication token and complete the login.
 
 Set the env variable `SERVICE_ACCOUNT_EMAIL` and mount the gcloud config into `/root/.config/gcloud` when executig the docker command: