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/input.ts

@@ -85,6 +85,10 @@ class Input {
     return core.getInput('customParameters') || '';
   }
 
+  static get sshAgent() {
+    return core.getInput('sshAgent') || '';
+  }
+
   static get chownFilesTo() {
     return core.getInput('chownFilesTo') || '';
   }