Fix issue/1312 - Use caching in docker build

[AlexChesser]

Per comment in issue #1312

the current state of the dockerfile in the aspnet/home repo does not use docker caching on DNU RESTORE (soon to be dotnet restore)

A small fix to the sample dockerfile can make the compile time of "second deploy" in docker 23x faster

old-speed for every deploy & first deploy under new method: ~3m30s
Start: 06:03:13.313
Finish: 06:06:40.640

new-speed - second & subsequent deploys if dependencies are unchanged: ~9s
Start: 06:00:36:036
Finish: 06:00:45:045

I also posted the FULL LOG of the difference over here: aspnet/aspnet-docker#123

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[glennc]

You can't do this unless the application you are deploying also has a .dockerignore file in order to avoid copying the lock file from the app into the image. Otherwise it will fail at runtime if you have restored on the host as well as in the image.

[Eilon]

@glennc any thoughts on this?

[glennc]

No longer relevant now that there are no samples here.