Fix handling of long username/password

Apparently the busybox implementation of `base64` will line-wrap long output strings.
This meant that long username+password combinations could produce base64 that
contained spurious "\n" characters, which then led to:
```
2019/05/06 00:47:39 Unable to parse "/kaniko/.docker/config.json": invalid character '\n' in string literal
```

Fixed by just removing the newlines in base64 output.  A "better" solution would use a different base64
implementation that avoided line-wrapping in the first place.

Dockerfile

@@ -1,7 +1,7 @@
 FROM gcr.io/kaniko-project/executor:debug-v0.7.0
 
 ENV HOME /root
-ENV USER /root
+ENV USER root
 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

plugin.sh

@@ -4,7 +4,7 @@ set -euo pipefail
 
 export PATH=$PATH:/kaniko/
 
-DOCKER_AUTH=`echo -n "${PLUGIN_USERNAME}:${PLUGIN_PASSWORD}" | base64`
+DOCKER_AUTH=`echo -n "${PLUGIN_USERNAME}:${PLUGIN_PASSWORD}" | base64 | tr -d "\n"`
 
 REGISTRY=${PLUGIN_REGISTRY:-https://index.docker.io/v1/}
 
@@ -21,10 +21,14 @@ DOCKERJSON
 DOCKERFILE=${PLUGIN_DOCKERFILE:-Dockerfile}
 CONTEXT=${PLUGIN_CONTEXT:-$PWD}
 LOG=${PLUGIN_LOG:-info}
-case "${PLUGIN_CACHE:-}" in
-  true) CACHE="true" ;;
-     *) CACHE="false" ;;
-esac
+
+if [[ -n "${PLUGIN_TARGET:-}" ]]; then
+    TARGET="--target=${PLUGIN_TARGET}"
+fi
+
+if [[ "${PLUGIN_CACHE:-}" == "true" ]]; then
+    CACHE="--cache=true"
+fi
 
 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)
@@ -39,6 +43,7 @@ fi
 /kaniko/executor -v ${LOG} \
     --context=${CONTEXT} \
     --dockerfile=${DOCKERFILE} \
-    --cache=${CACHE} \
     ${DESTINATIONS} \
+    ${CACHE:-} \
+    ${TARGET:-} \
     ${BUILD_ARGS:-}