docker: update alpine build enable set own version

docker: update alpine build enable set own version

Add ability to set your own env for the version of the docker
container alpine image. This is useful for applications like GNS3
who pin a specific version to look for when they boot up. When you build
locally to test your code you can just set the version to 0 so you don't
have to update configs/scripts looking for a specific image version.

Also fix a shebang in docker start for alpine.

Signed-off-by: Stephen Worley <sworley@nvidia.com>