Support for custom GOPROXY

[wburningham]

The dockers images are great to use, but for my private instance I also need to specify a private go proxy. Please add support for reading the following go ENV variables:

GOPRIVATE
GOPROXY
GONOPROXY
GONOSUMDB

On the following line:

go-playground/pkg/compiler/compiler.go

Line 48 in 5417f9a

cmd.Env = buildArgs

I can submit a PR if you want me to.

[x1unix]

@wburningham feel free to submit a PR!

As I understood, os environment variables should be merged with build args env vars