Kaniko needs better proxy handling

[tibers]

Actual behavior

If I setup proxies in my docker service definition, containers which kaniko spawns will ignore those proxy settings. This is really a go problem, as best I can tell. They will eventually fail with the error:

 ---> Running in dfd02818f95e
# Copyright 2018 Google LLC
# cd .; git clone https://github.com/awslabs/amazon-ecr-credential-helper /go/src/github.com/awslabs/amazon-ecr-credential-helper
Cloning into '/go/src/github.com/awslabs/amazon-ecr-credential-helper'...
fatal: unable to access 'https://github.com/awslabs/amazon-ecr-credential-helper/': Failed to connect to github.com port 443: Connection timed out
package github.com/awslabs/amazon-ecr-credential-helper/ecr-login/cli/docker-credential-ecr-login: exit status **128**```

**Expected behavior**
Proxies set in the docker configuration work with whatever go is doing.

**To Reproduce**
1. have a squid proxy
1. have docker behind the squid proxy
1. do something like `Environment="HTTP_PROXY=http://my.secret.proxy"` in the docker config
1. `go` doesn't honor it, nor lowercase notation.