ease disabling docker

[paulfantom]

This allows to easily disable dockerized build env by setting NO_DOCKER env variable to anything.

This is also needed for CI system.

Example run:

make build              # build in docker container (golang:1.10)
NO_DOCKER=1 make build  # build using host env

[openshift-ci-robot]

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[enxebre]

Cant' we just set DOCKER_CMD to point to the env variable value, otherwise to default
Otherwise with this approach you'd want to clean DOCKER_CMD when NO_DOCKER is set, if you don't and as a user I set env DOCKER_CMD to something and later I set NO_DOCKER=1, DOCKER_CMD still will run my previews command

[paulfantom]

Cant' we just set DOCKER_CMD to point to the env variable value, otherwise to default

I need something that unsets DOCKER_CMD and not sets it to something (not even empty string). Solution you propose, won't do it.
I was thinking about some other checks, but this is far simpler and I don't want to create some advanced Makefile which only I will know how it works.