Allow specifying builder environments in ACI manifest

[crackcomm]

This change let's use do something like that:

name: aci.github.io/aci-go-app
aci:
  dependencies:
    - aci.github.io/aci-libc
builder:
  dependencies:
    - aci.github.io/aci-builder-go
  environment:
    - { name: GOREPO, value: "github.com/crackcomm/cloudflare/cf" }

Specifying builder parameters by specifying environment variables.

[n0rad]

Thanks.

I also added a section in documentation on how to import dynamically env vars to builder :

To import dynamically environment vars to builder, you can use `dgr --set-env=TOTO=TITI`. This is usefull to import proxy settings without explicitly knowing the proxy inside the build process. 

Also all `DGR_ENV_*` vars are automatically imported. 
ex: having `export DGR_ENV_no_proxy=127.0.0.1`  on the host will end up having `no_proxy=127.0.0.1` in builder.