pkg/generator: add generation for docker image building scripts

[fanminshi]

add code generation for docker image build scripts. those are the scripts that operator-sdk build $IMAGE can use to create the operator image.

[fanminshi]

Manual Test:

$ operator-sdk new app-operator-2 --api-version=app.example.com/v1alpha1 --kind=AppService
$ cd app-operator-2/tmp/build/
$ cat docker_build.sh
#!/usr/bin/env bash

if ! which docker > /dev/null; then
	echo "docker needs to be installed"
	exit 1
fi

: ${IMAGE:?"Need to set IMAGE, e.g. gcr.io/<repo>/<your>-operator"}

echo "building container ${IMAGE}..."
docker build -t "${IMAGE}" -f tmp/build/Dockerfile .
fanmins-MBP:build fanminshi$ cat Dockerfile
FROM alpine:3.6

ADD tmp/_output/bin/app-operator-2 /usr/local/bin/app-operator-2

RUN adduser -D app-operator-2
USER app-operator-2
fanmins-MBP:build fanminshi$ cat Dockerfile
FROM alpine:3.6

ADD tmp/_output/bin/app-operator-2 /usr/local/bin/app-operator-2

RUN adduser -D app-operator-2
USER app-operator-2

[fanminshi]

cc/ @hasbro17 @hongchaodeng

[fanminshi]

cc/ @hasbro17 @hongchaodeng

[hasbro17]

LGTM