ci: install dcos-launch from git
ci: install dcos-launch from git

Adjust Dockerfile to install docs-launch from git instead of using a prebuilt  binary. This change is a hotfix to workaround issues with
the latest dcos-launch release as it broke out system tests.

dcos/dcos-ui,dcos/dcos-ui,dcos/dcos-ui,dcos/dcos-ui,dcos/dcos-ui