Add Ability To Skip Push

This adds the ability to build the image without pushing it by omitting
the `tags` and `repo` options.

Dockerfile

@@ -1,4 +1,4 @@
-FROM gcr.io/kaniko-project/executor:debug-v0.7.0
+FROM gcr.io/kaniko-project/executor:debug-v0.9.0
 
 ENV HOME /root
 ENV USER root

README.md

@@ -2,7 +2,7 @@
 
 A thin shim-wrapper around the official [Google Kaniko](https://cloud.google.com/blog/products/gcp/introducing-kaniko-build-container-images-in-kubernetes-and-google-container-builder-even-without-root-access) Docker image to make it behave like the [Drone Docker plugin](http://plugins.drone.io/drone-plugins/drone-docker/).
 
-Example .drone.yml for Drone 1.0
+Example .drone.yml for Drone 1.0 (pushing to Docker Hub):
 
 ```yaml
 kind: pipeline
@@ -12,7 +12,7 @@ steps:
 - name: publish
   image: banzaicloud/drone-kaniko
   settings:
-    registry: registry.example.com
+    registry: registry.example.com # if not provided index.docker.io is supposed
     repo: registry.example.com/example-project
     tags: ${DRONE_COMMIT_SHA}
     cache: true
@@ -25,6 +25,24 @@ steps:
       from_secret: docker-password
 ```
 
+Pushing to GCR:
+
+```yaml
+kind: pipeline
+name: default
+
+steps:
+- name: publish
+  image: banzaicloud/drone-kaniko
+  settings:
+    registry: gcr.io
+    repo: example.com/example-project
+    tags: ${DRONE_COMMIT_SHA}
+    cache: true
+    json_key:
+      from_secret: google-application-credentials
+```
+
 ## Test that it can build
 
 ```bash
@@ -54,12 +72,18 @@ docker build -t banzaicloud/drone-kaniko .
 Warm up the alpine image to the cache:
 
 ```bash
-docker run -v $PWD:/cache gcr.io/kaniko-project/warmer:latest --image=alpine:3.8
+docker run -v $PWD:/cache gcr.io/kaniko-project/warmer:latest --verbosity=debug --image=alpine:3.8
 ```
 
 
-Run the builder on the host network to be able to access the registry:
+Run the builder (on the host network to be able to access the registry, if any specified) with mounting the local disk cache, this example pushes to Docker Hub:
 
 ```bash
-docker run --net=host -it --rm -w /src -v $PWD:/cache -v $PWD:/src -e DOCKER_USERNAME=${DOCKER_USERNAME} -e DOCKER_PASSWORD=${DOCKER_PASSWORD} -e PLUGIN_REPO=banzaicloud/drone-kaniko-test -e PLUGIN_TAGS=test -e PLUGIN_DOCKERFILE=Dockerfile.test banzaicloud/drone-kaniko
+docker run --net=host -it --rm -w /src -v $PWD:/cache -v $PWD:/src -e PLUGIN_USERNAME=${DOCKER_USERNAME} -e PLUGIN_PASSWORD=${DOCKER_PASSWORD} -e PLUGIN_REPO=banzaicloud/drone-kaniko-test -e PLUGIN_TAGS=test -e PLUGIN_DOCKERFILE=Dockerfile.test -e PLUGIN_CACHE=true banzaicloud/drone-kaniko
+```
+
+The very same example just pushing to GCR instead of Docker Hub:
+
+```bash
+docker run --net=host -it --rm -w /src -v $PWD:/cache -v $PWD:/src -e PLUGIN_REGISTRY=gcr.io -e PLUGIN_REPO=paas-dev1/drone-kaniko-test -e PLUGIN_TAGS=test -e PLUGIN_DOCKERFILE=Dockerfile.test -e PLUGIN_CACHE=true -e PLUGIN_JSON_KEY="$(<$HOME/google-application-credentials.json)" banzaicloud/drone-kaniko
 ```

plugin.sh

@@ -4,9 +4,10 @@ set -euo pipefail
 
 export PATH=$PATH:/kaniko/
 
-DOCKER_AUTH=`echo -n "${PLUGIN_USERNAME}:${PLUGIN_PASSWORD}" | base64 | tr -d "\n"`
+REGISTRY=${PLUGIN_REGISTRY:-index.docker.io}
 
-REGISTRY=${PLUGIN_REGISTRY:-https://index.docker.io/v1/}
+if [ "${PLUGIN_USERNAME:-}" ] || [ "${PLUGIN_PASSWORD:-}" ]; then
+    DOCKER_AUTH=`echo -n "${PLUGIN_USERNAME}:${PLUGIN_PASSWORD}" | base64 | tr -d "\n"`
 
     cat > /kaniko/.docker/config.json <<DOCKERJSON
 {
@@ -17,6 +18,12 @@ cat > /kaniko/.docker/config.json <<DOCKERJSON
     }
 }
 DOCKERJSON
+fi
+
+if [ "${PLUGIN_JSON_KEY:-}" ];then
+    echo "${PLUGIN_JSON_KEY}" > /kaniko/gcr.json
+    export GOOGLE_APPLICATION_CREDENTIALS=/kaniko/gcr.json
+fi
 
 DOCKERFILE=${PLUGIN_DOCKERFILE:-Dockerfile}
 CONTEXT=${PLUGIN_CONTEXT:-$PWD}
@@ -30,14 +37,18 @@ if [[ "${PLUGIN_CACHE:-}" == "true" ]]; then
     CACHE="--cache=true"
 fi
 
-if [[ -n "${PLUGIN_BUILD_ARGS:-}" ]]; then
+if [ -n "${PLUGIN_BUILD_ARGS:-}" ]; then
     BUILD_ARGS=$(echo "${PLUGIN_BUILD_ARGS}" | tr ',' '\n' | while read build_arg; do echo "--build-arg=${build_arg}"; done)
 fi
 
-if [[ -n "${PLUGIN_TAGS:-}" ]]; then
-    DESTINATIONS=$(echo "${PLUGIN_TAGS}" | tr ',' '\n' | while read tag; do echo "--destination=${PLUGIN_REPO}:${tag} "; done)
-else
+if [ -n "${PLUGIN_TAGS:-}" ]; then
+    DESTINATIONS=$(echo "${PLUGIN_TAGS}" | tr ',' '\n' | while read tag; do echo "--destination=${REGISTRY}/${PLUGIN_REPO}:${tag} "; done)
+elif [ -n "${PLUGIN_REPO:-}" ]; then
     DESTINATIONS="--destination=${PLUGIN_REPO}:latest"
+else
+    DESTINATIONS="--no-push"
+    # Cache is not valid with --no-push
+    CACHE=""
 fi
 
 /kaniko/executor -v ${LOG} \