feat: output registry URI if single registry ID is provided as input

action.yml

@@ -9,7 +9,7 @@ inputs:
     required: false
 outputs:
   registry:
-    description: 'The URI of the default ECR registry i.e. aws_account_id.dkr.ecr.region.amazonaws.com, if none were provided as inputs'
+    description: 'The URI of the ECR registry i.e. aws_account_id.dkr.ecr.region.amazonaws.com. If multiple registries are provided as inputs, this output will not be set.'
 runs:
   using: 'node12'
   main: 'dist/index.js'

index.js

@@ -29,8 +29,8 @@ async function run() {
       const creds = authToken.split(':', 2);
       const proxyEndpoint = authData.proxyEndpoint;
 
-      if (!registries) {
-        // output the default registry if none were provided
+      if (authTokenResponse.authorizationData.length == 1) {
+        // output the registry URI if this action is doing a single registry login
         const registryId = proxyEndpoint.replace(/^https?:\/\//,'');
         core.setOutput('registry', registryId);
       }

index.test.js

@@ -85,6 +85,37 @@ describe('Login to ECR', () => {
             expect.anything());
     });
 
+    test('outputs the registry ID if a single registry is provided in the input', async () => {
+        core.getInput = jest.fn().mockReturnValueOnce('111111111111');
+        mockEcrGetAuthToken.mockImplementation(() => {
+            return {
+                promise() {
+                    return Promise.resolve({
+                        authorizationData: [
+                            {
+                                authorizationToken: Buffer.from('foo:bar').toString('base64'),
+                                proxyEndpoint: 'https://111111111111.dkr.ecr.aws-region-1.amazonaws.com'
+                            }
+                       ]
+                    });
+                }
+            };
+        });
+
+        await run();
+
+        expect(mockEcrGetAuthToken).toHaveBeenCalledWith({
+            registryIds: ['111111111111']
+        });
+        expect(core.setOutput).toHaveBeenCalledTimes(1);
+        expect(core.setOutput).toHaveBeenNthCalledWith(1, 'registry', '111111111111.dkr.ecr.aws-region-1.amazonaws.com');
+        expect(exec.exec).toHaveBeenCalledTimes(1);
+        expect(exec.exec).toHaveBeenNthCalledWith(1,
+            'docker login',
+            ['-u', 'foo', '-p', 'bar', 'https://111111111111.dkr.ecr.aws-region-1.amazonaws.com'],
+            expect.anything());
+    });
+
     test('error is caught by core.setFailed for failed docker login', async () => {
         exec.exec.mockReturnValue(1);