diff --git a/lib/manager/dockerfile/__snapshots__/extract.spec.ts.snap b/lib/manager/dockerfile/__snapshots__/extract.spec.ts.snap
index 41e944805da20ca09c3ac4a31536daab07df9fd6..0abb636aec22c8339de66d2ae62b6753c208c947 100644
--- a/lib/manager/dockerfile/__snapshots__/extract.spec.ts.snap
+++ b/lib/manager/dockerfile/__snapshots__/extract.spec.ts.snap
@@ -346,6 +346,22 @@ Array [
 ]
 `;
 
+exports[`manager/dockerfile/extract extractPackageFile() handles prefixes 1`] = `
+Array [
+  Object {
+    "autoReplaceStringTemplate": "{{lookupName}}{{#if newValue}}:{{newValue}}{{/if}}{{#if newDigest}}@{{newDigest}}{{/if}}",
+    "currentDigest": undefined,
+    "currentValue": "18.04",
+    "datasource": "docker",
+    "depName": "ubuntu",
+    "depType": "final",
+    "lookupName": "amd64/ubuntu",
+    "replaceString": "amd64/ubuntu:18.04",
+    "versioning": "ubuntu",
+  },
+]
+`;
+
 exports[`manager/dockerfile/extract extractPackageFile() handles quay hosts with port 1`] = `
 Object {
   "autoReplaceStringTemplate": "{{lookupName}}{{#if newValue}}:{{newValue}}{{/if}}{{#if newDigest}}@{{newDigest}}{{/if}}",
diff --git a/lib/manager/dockerfile/extract.spec.ts b/lib/manager/dockerfile/extract.spec.ts
index 896745048dfb3bd4f93f2e61f7a7603a840e227b..1e64e85dd7fc8ab2f5f46cb58474facd798f2aa0 100644
--- a/lib/manager/dockerfile/extract.spec.ts
+++ b/lib/manager/dockerfile/extract.spec.ts
@@ -265,6 +265,20 @@ describe('manager/dockerfile/extract', () => {
         },
       ]);
     });
+
+    it('handles prefixes', () => {
+      const res = extractPackageFile('FROM amd64/ubuntu:18.04\n').deps;
+      expect(res).toMatchSnapshot([
+        {
+          currentValue: '18.04',
+          depName: 'ubuntu',
+          lookupName: 'amd64/ubuntu',
+          versioning: 'ubuntu',
+          autoReplaceStringTemplate:
+            '{{lookupName}}{{#if newValue}}:{{newValue}}{{/if}}{{#if newDigest}}@{{newDigest}}{{/if}}',
+        },
+      ]);
+    });
   });
   describe('getDep()', () => {
     it('rejects null', () => {
diff --git a/lib/manager/dockerfile/extract.ts b/lib/manager/dockerfile/extract.ts
index a21cf10f9d16a8ab3315152f0ba0eea540df2047..4a0b6f11ec3f7583062cf8737e48a32d8646e2ac 100644
--- a/lib/manager/dockerfile/extract.ts
+++ b/lib/manager/dockerfile/extract.ts
@@ -58,6 +58,10 @@ export function getDep(
       if (dep.depName.startsWith(`${prefix}/`)) {
         dep.lookupName = dep.depName;
         dep.depName = dep.depName.replace(`${prefix}/`, '');
+        if (specifyReplaceString) {
+          dep.autoReplaceStringTemplate =
+            '{{lookupName}}{{#if newValue}}:{{newValue}}{{/if}}{{#if newDigest}}@{{newDigest}}{{/if}}';
+        }
       }
     }
   }
diff --git a/lib/manager/droneci/__snapshots__/extract.spec.ts.snap b/lib/manager/droneci/__snapshots__/extract.spec.ts.snap
index 0ed8a05d30d5d5a38da601986c6eeda3773e7e12..a7fe8577091daaea9398a0abdd2b679013ced919 100644
--- a/lib/manager/droneci/__snapshots__/extract.spec.ts.snap
+++ b/lib/manager/droneci/__snapshots__/extract.spec.ts.snap
@@ -12,7 +12,7 @@ Array [
     "replaceString": "elixir:1.8.1-alpine",
   },
   Object {
-    "autoReplaceStringTemplate": "{{depName}}{{#if newValue}}:{{newValue}}{{/if}}{{#if newDigest}}@{{newDigest}}{{/if}}",
+    "autoReplaceStringTemplate": "{{lookupName}}{{#if newValue}}:{{newValue}}{{/if}}{{#if newDigest}}@{{newDigest}}{{/if}}",
     "currentDigest": "sha256:36adc17e9cceab32179d3314da9cb9c737ffb11f0de4e688f407ad6d9ca32201",
     "currentValue": "10.0.0",
     "datasource": "docker",