Using SSH_AUTH_SOCK (ssh agent forwarding) to pull upm private repos (#256)

* using SSH_AUTH_SOCK (ssh agent forwarding) to pull upm private repos

* sshAgent as input parameter

* yarn run prettier --write "src/**/*.{js,ts}"

* yarn run lint --fix && yarn build

* fixed compilation after rebase

* removed RUN apt-get update && apt-get install -y openssh-client. This change needs to be done upstream. See game-ci/docker#117

src/model/__mocks__/input.ts

@@ -10,6 +10,7 @@ export const mockGetFromUser = jest.fn().mockResolvedValue({
   buildMethod: undefined,
   buildVersion: '1.3.37',
   customParameters: '',
+  sshAgent: '',
   chownFilesTo: '',
 });