This commit reworks Dockerfile.linux to make it more in-line with Docker
and Linux best practices.
* Set entrypoint to acs-engine
* Set default parameters to show acs-engine help
* Set default ACSENGINE_VERSION to latest release
* Remove unnecessary curl install
* Remove unnecessary bash install
* Use /opt/ as intall dir rather than root home
* Newer version of alpine
* Update README to use latest release version