Dockerfile

[jgolob]

Modified the dockerfile to also generate a docker container useful for using and building pplacer. Built off the current master branch of the origin repo (to incorporate other changes to pplacer). Directly loads file from repo rather than git pull. Zips binaries into /pplacer.zip to ease update of binary distros. Installs scripts into container environment.

[matsen]

Thanks, @jgolob !

See how many commits are part of your PR? It'd be nicer if you'd rebase off of master (and then squash). Or, given that this is just one file, you can open a separate PR just adding the Dockerfile.

[jgolob]

As you've suggested, I've rebased off of master.