Support documented rules

index.js

@@ -1,23 +1,40 @@
 const core = require('@actions/core');
 
+/**
+https://docs.docker.com/engine/reference/commandline/tag/#description
+Name components may contain lowercase letters, digits and separators.
+A separator is defined as a period, one or two underscores, or one or
+more dashes. A name component may not start or end with a separator.
+**/
+
 try {
   const path = core.getInput('path');
-  const prefix = core.getInput('prefix')
-  let result = path
+  const prefix = core.getInput('prefix');
+  let result = path;
 
   // Remove prefix
   if (path.startsWith(prefix)) {
-    result = result.substr(prefix.length)
+    result = result.substr(prefix.length);
   }
 
-  // Replace '/'
-  result = result.replaceAll('/', '-')
+  // Remove start 
+  result = result.replace(/^\./, '');
 
+  // Replace '/' (we expect lots of / in paths)
+  result = result.replaceAll('/', '-');
 
-  // Remove start and end path characters
-  result = result.replace(/^\./, '')
-  result = result.replace(/^-/, '')
-  result = result.replace(/-$/, '')
+  // Replace excessive underscores
+  result = result.replaceAll(/___+/, '__');
+
+  // Lower case
+  result = result.toLowerCase();
+
+  // Remove invalid characters
+  result = result.replaceAll(/[^a-z0-9\._-]/, '')
+
+  // Remove start and end seperators
+  result = result.replace(/^[-\._]+/, '')
+  result = result.replace(/[-\._]+$/, '')
 
   core.setOutput('image-name', result);