Add dockerCommandTemplate to Config. (DefinitelyTyped#37984)

https://github.com/garris/BackstopJS#using-docker-for-testing-across-different-environments

Author: cdeutsch

types/backstopjs/backstopjs-tests.ts

@@ -18,7 +18,9 @@ backstop('test', {
     id: 'foo',
     scenarios: [],
     viewports: [],
-  }
+    dockerCommandTemplate:
+      'docker run --rm -it --net="host" --mount type=bind,source="{cwd}",target=/src backstopjs/backstopjs:{version} {backstopCommand} {args}',
+  },
 });
 
 /** Custom example */

types/backstopjs/index.d.ts

@@ -14,6 +14,7 @@ export interface Config {
   };
   debugWindow?: boolean;
   debug?: boolean;
+  dockerCommandTemplate?: string;
   engine?: 'chromy' | 'puppeteer';
   engineOptions?: {
     args: string[];