add ability to enter arbitrary Dockerfile instructions

[kaczmarj]

A class like ANTs would benefit from the option of supplying a URL with the latest binaries.

[kaczmarj]

To add to this, create a more general function to download binaries, untar, and add to PATH. Parameters could be URL to binaries and the path to add to PATH. Could assume that untarred directory will have the same name as the tarball, but optional parameter could be used to specify the name of the untarred directory.

[kaczmarj]

Instead of adding an option to use custom URL, create an interface that lets users add arbitrary Dockerfile commands.