Handle issue with GoogleContainerTools/kaniko#1209

To get around GoogleContainerTools/kaniko#1209 we need to auth using the
DockerHub v1 API. As that variable is the "registry" and we have to use
a FQDN with Protocol we have to convert to standard registry format for
tagging the image, otherwise there's an issue with the auth mechanism
where it attempts to normalize the URL and incorrectly parses it.

GoogleContainerTools/kaniko#1209

plugin.sh

@@ -4,7 +4,7 @@ set -euo pipefail
 
 export PATH=$PATH:/kaniko/
 
-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"`
@@ -20,6 +20,12 @@ if [ "${PLUGIN_USERNAME:-}" ] || [ "${PLUGIN_PASSWORD:-}" ]; then
 DOCKERJSON
 fi
 
+# If we're using dockerhub we have to convert from URL with protocol to
+# standard format for use throughout the rest of the system
+if [ "${REGISTRY}" == "https://index.docker.io/v1/" ]; then
+  REGISTRY="docker.io"
+fi
+
 if [ "${PLUGIN_JSON_KEY:-}" ];then
     echo "${PLUGIN_JSON_KEY}" > /kaniko/gcr.json
     export GOOGLE_APPLICATION_CREDENTIALS=/kaniko/gcr.json