Add build_args support

Dockerfile

@@ -1,4 +1,7 @@
-FROM gcr.io/kaniko-project/executor:v0.6.0 AS kaniko
+# TODO use the original image, but currently this is a blocker:
+# https://github.com/GoogleContainerTools/kaniko/pull/459
+FROM banzaicloud/kaniko:latest AS kaniko
+# FROM gcr.io/kaniko-project/executor:v0.6.0 AS kaniko
 
 FROM alpine:3.8
 
@@ -10,6 +13,8 @@ ENV SSL_CERT_DIR=/kaniko/ssl/certs
 ENV DOCKER_CONFIG /kaniko/.docker/
 ENV DOCKER_CREDENTIAL_GCR_CONFIG /kaniko/.config/gcloud/docker_credential_gcr_config.json
 
+RUN apk add --update --no-cache jq
+
 # add the wrapper which acts as a drone plugin
 COPY plugin.sh /usr/bin/
 ENTRYPOINT [ "/usr/bin/plugin.sh" ]

plugin.sh

@@ -20,8 +20,10 @@ DOCKERFILE=${PLUGIN_DOCKERFILE:-Dockerfile}
 DESTINATION=${PLUGIN_REPO}:${PLUGIN_TAGS:-latest}
 CONTEXT=${PLUGIN_CONTEXT:-$PWD}
 LOG=${PLUGIN_LOG:-info}
+BUILD_ARGS=`echo ${PLUGIN_BUILD_ARGS:-} | jq -r 'map("--build-arg " + .) | join(" ")'`
 
 /kaniko/executor -v ${LOG} \
     --context ${CONTEXT} \
     --dockerfile ${DOCKERFILE} \
-    --destination ${DESTINATION}
+    --destination ${DESTINATION} \
+    ${BUILD_ARGS}