Makefile Recipe: Build with Docker and Export To A Gzipped Tarball

I have been writing a lot of Makefiles lately. I find them simple and easy to like. And, as with all old-school things, I am starting to overlook its quirks.

I needed to write a Makefile that lets me build something with Docker and then export the entire contents of the image to a gzipped tarball.

The first part is easy:

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.PHONY: build
build:
  (cd $(APP); docker build -t thing-$(APP):latest .)

When run as make build APP=something, it will build the Dockerfile within the something/ directory.

But the next part is where it gets a bit interesting.

To export the contents of the image, as if it were a root filesystem, into a tarball, we need to create a container first. We need to run a few docker commands back to back, so we need to know the container name (or ID).

In Makefile:

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.PHONY: tarball
tarball:
  TS=$$(date +'%s') && \
  docker create --name=tmp-$(APP)-$$TS tmp-$(APP):latest && \
  docker export tmp-$(APP)-$$TS | gzip > $(APP).tar.gz && \
  docker rm tmp-$(APP)-$$TS

This script will first set the current Unix timestamp to TS and then:

• Create a Docker container named “tmp-{APP}-{TS}”, where {APP} and {TS} will be interpolated.
• Export the contents of the Docker container (which is a tarball), pipe it through gzip and then save it as a file named {APP}.tar.gz.
• Finally, it will remove the Docker container.

The script could be better. But it is a good start for what I needed. And, as with all things, it can be improved as and when necessary.

This post is 29th of my #100DaysToOffload challenge. Want to get involved? Find out more at 100daystooffload.com.