fix: filter out null docker digests

Closes #1739

lib/manager/docker/package.js

@@ -27,7 +27,11 @@ async function getPackageUpdates(config) {
       currentTag
     );
     if (!newDigest) {
-      logger.debug({ content: config.content }, 'Dockerfile no digest');
+      logger.warn(
+        { dockerRegistry, depName, currentTag },
+        'Dockerfile no digest'
+      );
+      return [];
     }
     if (newDigest && config.currentDigest !== newDigest) {
       const upgrade = {};
@@ -144,6 +148,12 @@ async function getPackageUpdates(config) {
           config.depName,
           upgrade.newTag
         );
+        if (!upgrade.newDigest) {
+          logger.warn(
+            { dockerRegistry, depName, tag: upgrade.newTag },
+            'Dockerfile no digest'
+          );
+        }
         upgrade.newFrom += `@${upgrade.newDigest}`;
       }
       if (newVersionMajor > currentMajor) {
@@ -160,7 +170,7 @@ async function getPackageUpdates(config) {
       );
     }
   }
-  return upgrades;
+  return upgrades.filter(u => u.newDigest !== null);
 }
 
 function isStable(tag, unstablePattern) {