generated, parameterized Dockerfiles

generate-dockerfile.sh

#!/bin/bash
set -e

cat <<-'EOF'
	# WARNING: This file is generated.  DO NOT MODIFY DIRECTLY.

	FROM ubuntu:12.04
	RUN ...
EOF

if [ "$SOME_KNOB" ]; then
	cat <<-'EOF'
		RUN ...
		RUN ...
	EOF
else
	cat <<-'EOF'
		RUN ...
		RUN ...
		ENV ...
	EOF
fi

cat <<-'EOF'
	RUN ...
	ENV ...
	RUN ...
	CMD [...]
EOF

Makefile

# ...
build:
	./hack/generate-dockerfile.sh > Dockerfile
	docker build -t docker .
# ...