Added ability to specify docker tag

Author: toote

hooks/command

@@ -15,8 +15,10 @@ args=(
   "--env"    "PLUGIN_ID=$BUILDKITE_PLUGIN_PLUGIN_LINTER_ID"
 )
 
+TAG=${BUILDKITE_PLUGIN_PLUGIN_LINTER_IMAGE_VERSION:-v2.0.1}
+
 if [[ -n "${BUILDKITE_PLUGIN_PLUGIN_LINTER_README:-}" ]] ; then
   args+=("--env" "PLUGIN_README=$BUILDKITE_PLUGIN_PLUGIN_LINTER_README")
 fi
 
-docker run "${args[@]}" buildkite/plugin-linter
+docker run "${args[@]}" buildkite/plugin-linter:"${TAG}"