Add "windowsservercore" variant

[tianon]

Closes #92 (commits included here)

Here's a snippet of the updated generate-stackbrew-library.sh output:

...

Tags: 1.6.3-alpine, 1.6-alpine, 1-alpine, alpine
GitCommit: 9f666dc2f4f51df564613f787d28b3a2353243e0
Directory: 1.6/alpine

Tags: 1.6.3-windowsservercore, 1.6-windowsservercore, 1-windowsservercore, windowsservercore
GitCommit: a30dac9f2492855f5040cb67de95d7181eebc9b8
Directory: 1.6/windows/windowsservercore
Constraints: windowsservercore

Tags: 1.7rc4, 1.7
GitCommit: cdf73bb71f9a0c0f9aa22beb3d9d34274aaf1b03
Directory: 1.7

...

[tianon]

I've used the following Dockerfile to test this (installing choco to install git to go get github.com/constabulary/gb) successfully:

FROM golang:windowsservercore

RUN Invoke-WebRequest https://chocolatey.org/install.ps1 -UseBasicParsing | Invoke-Expression

RUN choco install -y -params /GitAndUnixToolsOnPath git.install

RUN go get -v github.com/constabulary/gb/...; \
    gb help;

A few size notes:

• microsoft/windowsservercore: ~7.764 GB
• golang:windowsservercore: ~8.211 GB (so Go itself is ~0.447 GB)
• choco: ~8.279 GB (so choco is ~0.068 GB)
• git.install: ~8.808 GB (so Git itself is ~0.529 GB -- yes, bigger than Go)
• go get .../gb/...: ~8.864 GB

So I'm very strongly -1 on including Git by default, even though it hampers go get usage slightly (especially given that it really isn't all that hard to write those two lines to install choco and then git).

[yosifkit]

I am going to argue for including git:

• it is included in all* golang images
  • we even include bzr, mercurial, and subversion
• we include build tools like gcc/g++, and make too

So, are we going to treat the 7.7GB base windows image like the alpine tags and force users to sideload the tools they actually need? Maybe we could skip used tools like git in the nanoserver images?

Possibly useful links to install git without resorting to choco:

• http://superuser.com/questions/944576/git-for-windows-silent-install-silent-arguments
• https://chocolatey.org/packages/git.install#files (ie, just steal the portions of their script for what we need)

_{* except alpine based images}

[tianon]

Fair point -- so windowsservercore is the "full-featured" version, and nanoserver is the slim variant. Seems sane to me, and matches how nanoserver is generally described. 👍

(Probably worth noting that we're still working on a way to build official nanoserver-based images, which is why they are not included here.)

[tianon]

Ok, updated to install Git (without installing choco, which makes our Dockerfile a fair bit more verbose, but IMO is worth it to leave the unrelated choco as a choice for our users instead).

[yosifkit]

LGTM

[tianon]

Step 1, ✅
Step 2, docker-library/official-images#2051