dockerfilepp-labels

dockerfilepp-labels is a trivial go application which takes a Dockerfile on stdin and simply replaces some pre-defined values. The idea is to make Dockerfile declarative again, making multiple Dockerfiles doing the same thing easier to maintain.

The examples centre around populating dynamic label values, but this is for demonstration purposes only. You could imagine building your own library of DSL extensions in a similar way, or extending into a general purpose tool. For this purpose most of the work has been split out into a sepatatelibrary at github.com/garethr/dockerfile.

Problem

Dockerfile is wonderfully simple when it comes to hello-world examples, but the line-orientated nature and evolving best practices mean than it's common for some quite crazy imperative bash juggling to make it's way into what is best suited to a declarative build description. See the best practice for installing debian packages if you don't believe me.

The same hoops are often jumped through in multiple Dockerfiles, so the complex implementation details are copied and pasted into many places, making maintenance more costly.

Usage

So, given the following Dockerfile. We note that it is:

• Concise and declarative
• Totally not going to work if you pass it to docker because of the
  non-standard instructions.

FROM alpine:3.4
MAINTAINER Gareth Rushgrove "[email protected]"

DATETIME_LABEL
VCS_LABEL

So lets run that through dockerfilepp-labels. You can download a suitable release from the Releases list on GitHub, or build your own from this repo by running make build.

Once you have the binary you can use it like so:

cat Dockerfile | ./dockerfilepp-labels

This should output to stdout with a new Dockerfile which is going to work as an input to docker build

FROM alpine:3.4
MAINTAINER Gareth Rushgrove "[email protected]"

LABEL com.example.datetime="2016-09-06T10:53:35+01:00"
LABEL com.example.vcs-ref="da3a8068c237137b3b468445b482cc307545af4e"