Add ARGs for cross-compiling

[sixeyed]

Add build args. Defaults to Linux/x64 so no change to existing image, but can build for other platforms - e.g.

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

(Replaces #2209).

[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 "patch-1" [email protected]:sixeyed/distribution.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

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

[codecov]

Codecov Report

Merging #2281 into master will decrease coverage by 10.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #2281       +/-   ##
==========================================
- 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%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a528cc1...646fc97. Read the comment docs.

[sixeyed]

@stevvooe - this replaces #2209. Couldn't get my history into line :)

[dmcgowan]

LGTM

[stevvooe]

LGTM