Add build-docker and test-docker target to build without local Golang

[StefanScherer]

This may be helpful for community to get started compiling the plugin and start sending pull requests without the need to setup Golang on the local machine.

My Golang setup on my MacBook is quite old, I tried several first beginners steps and I just didn't want to touch it right now. I wanted to send a PR for #1471 and compile and test the plugin for my MacBook.

These two additional make targets allow users to build and test the plugin within a Linux Docker container with eg. Docker4Mac (and probably Docker4Windows).

Compiling the Linux version of the plugin:

$ make build-docker
mkdir -p bin
docker run --rm -v $(pwd)/bin:/go/bin -v $(pwd):/go/src/github.com/terraform-providers/terraform-provider-azurerm -w /go/src/github.com/terraform-providers/terraform-provider-azurerm -e GOOS golang:1.10 make build
==> Checking that code complies with gofmt requirements...
go install

$ ls bin
terraform-provider-azurerm

Compiling the macOS version of the plugin:

$ GOOS=darwin make build-docker
mkdir -p bin
docker run --rm -v $(pwd)/bin:/go/bin -v $(pwd):/go/src/github.com/terraform-providers/terraform-provider-azurerm -w /go/src/github.com/terraform-providers/terraform-provider-azurerm -e GOOS golang:1.10 make build
==> Checking that code complies with gofmt requirements...
go install

$ ls bin/darwin_amd64/
terraform-provider-azurerm

[tombuildsstuff]

LGTM - thanks for this 👍

[ghost]

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