pass environment variables to go build command of build_gcsfuse to support builds behind an HTTP proxy

[OutSorcerer]

Description

The problem I faced is described in the title, I hope this fix can be useful to someone else too.

I have a custom Dockerfile based on the original Dockerfile in this repo, and my build runs behind an HTTP proxy, the build was failing. I realized it was due the following code snippet in tools/build_gcsfuse/main.go:

cmd := exec.Command(
	"go",
	"build",
	...
			
// Set up environment.
cmd.Env = []string{
	"GO15VENDOREXPERIMENT=1",
	"GO111MODULE=auto",
	fmt.Sprintf("PATH=%s", pathEnv),
	fmt.Sprintf("GOROOT=%s", runtime.GOROOT()),
	fmt.Sprintf("GOPATH=%s", gopath),
	fmt.Sprintf("GOCACHE=%s", gocache),
	"CGO_ENABLED=0",
}			

This code ignores all the current environment variables, which is IMHO unexpected as it runs inside Docker which already takes care about creating an isolated environment with fresh values of environment variables.

The particular variable important for my case was https_proxy, without it go build was failing as it was unable to access internet directly.

That variable was passed with ~/.docker/config.json like described in Docker docs.

Link to the issue in case of a bug fix.

NA

Testing details

1. Manual - NA
2. Unit tests - NA
3. Integration tests - NA

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