Add LABEL directives to Dockerfile

Add LABEL directives to Dockerfile

This is so GitHub can detect this is a GitHub Action.

diff --git a/Dockerfile b/Dockerfile
index 45bdedd..5ad6cc0 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,5 +1,13 @@
 FROM ruby:2.6.0
 
+LABEL "name"="Publish to Rubygems"
+LABEL "version"="1.0.0"
+
+LABEL "com.github.actions.name"="Publish to Rubygems"
+LABEL "com.github.actions.icon"="upload"
+LABEL "com.github.actions.color"="red"
+LABEL "com.github.actions.description"="Build and publish your gem to Rubygems"
+
 ENV LC_ALL C.UTF-8
 ENV LANG en_US.UTF-8
 ENV LANGUAGE en_US.UTF-8

GitHub sha: 94936359