Allow skip-tls-verify option for testing

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

README.md

@@ -12,10 +12,11 @@ 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: ${DRONE_COMMIT_SHA}
     cache: true
+    skip_tls_verify: false # set to true for testing registries ONLY with self-signed certs
     build_args:
     - COMMIT_SHA=${DRONE_COMMIT_SHA}
     - COMMIT_AUTHOR_EMAIL=${DRONE_COMMIT_AUTHOR_EMAIL}
@@ -35,10 +36,11 @@ steps:
 - name: publish
   image: banzaicloud/drone-kaniko
   settings:
-    repo: gcr.io/example.com/example-project
+    registry: gcr.io
+    repo: example.com/example-project
     tags: ${DRONE_COMMIT_SHA}
     cache: true
-    google_application_credentials:
+    json_key:
       from_secret: google-application-credentials
 ```
 
@@ -84,5 +86,5 @@ docker run --net=host -it --rm -w /src -v $PWD:/cache -v $PWD:/src -e PLUGIN_USE
 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 $PWD:/src -e PLUGIN_REPO=gcr.io/banzaicloud/drone-kaniko-test -e PLUGIN_TAGS=test -e PLUGIN_DOCKERFILE=Dockerfile.test -e PLUGIN_CACHE=true -e PLUGIN_GOOGLE_APPLICATION_CREDENTIALS="$(<$HOME/google-application-credentials.json)" banzaicloud/drone-kaniko
+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

@@ -4,11 +4,11 @@ set -euo pipefail
 
 export PATH=$PATH:/kaniko/
 
+REGISTRY=${PLUGIN_REGISTRY:-index.docker.io}
+
 if [ "${PLUGIN_USERNAME:-}" ] || [ "${PLUGIN_PASSWORD:-}" ]; then
     DOCKER_AUTH=`echo -n "${PLUGIN_USERNAME}:${PLUGIN_PASSWORD}" | base64 | tr -d "\n"`
 
-    REGISTRY=${PLUGIN_REGISTRY:-https://index.docker.io/v1/}
-
     cat > /kaniko/.docker/config.json <<DOCKERJSON
 {
     "auths": {
@@ -20,19 +20,24 @@ if [ "${PLUGIN_USERNAME:-}" ] || [ "${PLUGIN_PASSWORD:-}" ]; then
 DOCKERJSON
 fi
 
-if [ "${PLUGIN_GOOGLE_APPLICATION_CREDENTIALS:-}" ];then
-    echo "${PLUGIN_GOOGLE_APPLICATION_CREDENTIALS}" > /kaniko/gcr.json
+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:-$PWD}
 LOG=${PLUGIN_LOG:-info}
+EXTRA_OPTS=""
 
 if [[ -n "${PLUGIN_TARGET:-}" ]]; then
     TARGET="--target=${PLUGIN_TARGET}"
 fi
 
+if [[ "${PLUGIN_SKIP_TLS_VERIFY:-}" == "true" ]]; then
+    EXTRA_OPTS="--skip-tls-verify=true"
+fi
+
 if [[ "${PLUGIN_CACHE:-}" == "true" ]]; then
     CACHE="--cache=true"
 fi
@@ -42,14 +47,19 @@ if [ -n "${PLUGIN_BUILD_ARGS:-}" ]; then
 fi
 
 if [ -n "${PLUGIN_TAGS:-}" ]; then
-    DESTINATIONS=$(echo "${PLUGIN_TAGS}" | tr ',' '\n' | while read tag; do echo "--destination=${PLUGIN_REPO}:${tag} "; done)
-else
+    DESTINATIONS=$(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 ${LOG} \
     --context=${CONTEXT} \
     --dockerfile=${DOCKERFILE} \
+    ${EXTRA_OPTS} \
     ${DESTINATIONS} \
     ${CACHE:-} \
     ${TARGET:-} \