`SKIP_DOCKER=true make build` still attempts to use Docker for Darwin build about duffle

[krancour]

@radu-matei

I believe this is doing what it is supposed to. The build target is defined as follows:

build: build-all-bins build-image

The pre-req build-all-bins target is happening outside of Docker:

bash -c "LDFLAGS=\"-w -s -X github.com/cnabio/duffle/pkg/version.Version=2fc6ad7\" scripts/build.sh"

The pre-req build-image target is a different story. It's building a Docker image-- which cannot be accomplished without docker build (unless you want to bring something like kaniko into the mix).

SKIP_DOCKER=true really only stops things that can be done without Docker from being done without Docker. Building an image isn't one of those things.