Update hack/make.sh with slightly clearer output and a more precise "are we in the Docker container?" check

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Make get.docker.io install directions slightly better by telling user exactly how to add themselves to the "docker" group

As a bonus, if the user has run this script the way we recommend (ie, without "sudo" or "su", run as their user), we can actually give them the exact command they'll need with their correct username substituted appropriately.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
 Fix fun Travis DCO check YAML parsing issues (especially with commit messages that start with any kind of whitespace, like this one intentionally does)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
vagrant: added PRIVATE_NETWORK env var

Docker-DCO-1.1-Signed-off-by: Rafal Jeczalik <rjeczalik@gmail.com> (github: rjeczalik)
Merge pull request #3848 from tianon/fix-yaml-space-parsing

 Fix fun Travis DCO check YAML parsing issues (especially with commit messages that start with any kind of whitespace, like this one intentionally does)
Merge pull request #3760 from tianon/better-nondocker-warning

Update hack/make.sh with slightly clearer output and a more precise "are we in the Docker container?" check
Merge pull request #3822 from tianon/install-your-user

Make get.docker.io install directions slightly better by telling user exactly how to add themselves to the "docker" group
Merge pull request #3125 from rjeczalik/master

vagrant: added PRIVATE_NETWORK env var
Fix ADD remote files issue. Hitting the cache for different file's content (#3847)

Force reading of downloaded file contents to obtain the checksum used by cache validation.

Docker-DCO-1.1-Signed-off-by: German Del Zotto <germ@ndz.com.ar> (github: GermanDZ)
Rewrite with a simpler approach (just sum the downloaded file during the copy)

Docker-DCO-1.1-Signed-off-by: Abel Muiño <amuino@gmail.com> (github: amuino)