Update Dockerfile calling syntax

[glennc]

This is for #936

@anurse and @BrennanConroy Give me a shipit on the Dockerfile and general idea?

Issues with existing setup.

• Latest sample is supposed to always run against the latest DNX, not beta7. So it needs to run against the latest unstable.
• uname detection for clr fallback doesn't work on wheezy based Ubuntu. So I am going from Ubuntu 14.04 directly.
• Copying project.json, restoring, then copying app would overwrite the project.lock.json with one from my build context. Which would not work because it would overwrite the lock file that was created in restore with the one from the context. This probably doesn't break people if everything lines up 100% but is not really correct.
• Need NuGet.Config in a location that restore will use so that it gets packages from aspnetvnext instead of NuGet.org as it is running against nightly builds.

[dnfclas]

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[analogrelay]

Looks like a Dockerfile to me