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

@@ -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}

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
@@ -43,13 +48,18 @@ fi
 
 if [ -n "${PLUGIN_TAGS:-}" ]; then
     DESTINATIONS=$(echo "${PLUGIN_TAGS}" | tr ',' '\n' | while read tag; do echo "--destination=${REGISTRY}/${PLUGIN_REPO}:${tag} "; done)
-else
+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:-} \