fix(toolkit): correctly pass build args to docker build (#2634)

The option name (`--build-arg`) and value should be passed as separate array items.

Author: jogold

packages/aws-cdk/lib/docker.ts

@@ -57,9 +57,10 @@ export async function prepareContainerAsset(asset: ContainerImageAssetMetadataEn
       }
     }
 
+    const buildArgs = ([] as string[]).concat(...Object.entries(asset.buildArgs || {}).map(([k, v]) => ['--build-arg', `${k}=${v}`]));
     const baseCommand = [
       'docker', 'build',
-      ...Object.entries(asset.buildArgs || {}).map(([k, v]) => `--build-arg ${k}=${v}`),
+      ...buildArgs,
       '--tag', latest,
       asset.path
     ];

packages/aws-cdk/test/test.docker.ts

@@ -92,7 +92,7 @@ export = {
     }
 
     // THEN
-    const command = ['docker', 'build', '--build-arg a=b', '--build-arg c=d', '--tag', `uri:latest`, '/foo'];
+    const command = ['docker', 'build', '--build-arg', 'a=b', '--build-arg', 'c=d', '--tag', `uri:latest`, '/foo'];
     test.ok(shellStub.calledWith(command));
 
     prepareEcrRepositoryStub.restore();