Add ARGs for cross compiling

[sixeyed]

Bump Go to 1.8 and add build args. Defaults to Linux/x64 but can build for other platforms - e.g.

docker build --build-arg GOOS=windows -t distribution-builder:windows .

[codecov-io]

Codecov Report

Merging #2209 into master will decrease coverage by 10.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #2209       +/-   ##
==========================================
- Coverage   61.57%   51.5%   -10.08%     
==========================================
  Files         125     125               
  Lines       11397   11397               
==========================================
- Hits         7018    5870     -1148     
- Misses       3493    4781     +1288     
+ Partials      886     746      -140

Impacted Files	Coverage Δ
registry/storage/driver/gcs/gcs.go	0.4% <0%> (-68.94%)	⬇️
registry/storage/driver/oss/oss.go	0.56% <0%> (-57.23%)	⬇️
registry/storage/driver/s3-aws/s3.go	4.57% <0%> (-56.23%)	⬇️
registry/storage/driver/s3-goamz/s3.go	0.5% <0%> (-51.4%)	⬇️
registry/client/transport/transport.go	69.69% <0%> (-9.1%)	⬇️

---

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[stevvooe]

I'm not sure we are quite on 1.8 yet.

[stevvooe]

Thanks for the fix!

We aren't quite on 1.8 yet, so if you could leave the Dockerfile on 1.7, we can get this merged.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "master" git@github.com:sixeyed/distribution.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354572560
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[dmcgowan]

That update button doesn't work correctly, need to rebase locally and push up

[sixeyed]

By now we're on Alpine 1.8, so just the ARG lines added.

[stevvooe]

@sixeyed Could you fix these to remove the merge commits?

[sixeyed]

My fork is badly diverged... Closing this and will open a new PR with a clean merge history.