fix: destination lack

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-v0.9.0
+FROM gcr.io/kaniko-project/executor:debug-v0.10.0
 
 ENV HOME /root
 ENV USER root

README.md

@@ -16,6 +16,7 @@ steps:
     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}
@@ -43,6 +44,35 @@ steps:
       from_secret: google-application-credentials
 ```
 
+## Use `.tags` file for tagging
+
+Similarily to official
+[drone-docker](https://github.com/drone-plugins/drone-docker) plugin you can use
+`.tags` file to embed some custom logic for creating tags for an image.
+
+```yaml
+kind: pipeline
+name: default
+
+steps:
+- name: build
+  image: golang
+  commands:
+      - go get 
+      - go build
+      - make versiontags > .tags
+- name: publish
+  image: banzaicloud/drone-kaniko
+  settings:
+    registry: registry.example.com 
+    repo: registry.example.com/example-project
+    # tags: ${DRONE_COMMIT_SHA} <= it must be left undefined 
+    username:
+      from_secret: docker-username
+    password:
+      from_secret: docker-password
+```
+
 ## Test that it can build
 
 ```bash

plugin.sh

@@ -28,11 +28,16 @@ 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
@@ -41,16 +46,28 @@ 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 "${PLUGIN_BUILD_ARGS_FROM_ENV:-}" ]; then
+    BUILD_ARGS_FROM_ENV=$(echo "${PLUGIN_BUILD_ARGS_FROM_ENV}" | tr ',' '\n' | while read build_arg; do echo "--build-arg ${build_arg}=$(eval "echo \$$build_arg")"; done)
+fi
+
 if [ -n "${PLUGIN_TAGS:-}" ]; then
     DESTINATIONS=$(echo "${PLUGIN_TAGS}" | tr ',' '\n' | while read tag; do echo "--destination=${REGISTRY}/${PLUGIN_REPO}:${tag} "; done)
+elif [ -f .tags ]; then
+    DESTINATIONS=$(cat .tags| tr ',' '\n' | while read tag; do echo "--destination=${REGISTRY}/${PLUGIN_REPO}:${tag} "; done)
+elif [ -n "${PLUGIN_REPO:-}" ]; then
+    DESTINATIONS="--destination=${REGISTRY}/${PLUGIN_REPO}:latest"
 else
-    DESTINATIONS="--destination=${PLUGIN_REPO}:latest"
+    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:-} \
-    ${BUILD_ARGS:-}
+    ${BUILD_ARGS:-} \
+    ${BUILD_ARGS_FROM_ENV:-}