Containerising a tool

Portability and reproducibility are important aspects of janis. Building and using containers for your tool is a great way to ensure compatibility and portability across institutes.

Getting started

In this tutorial, we’re going to introduce containers and go through different ways to find or build containers for your tool.

What you’ll need

You’ll need an installation of Docker, and a Dockerhub account for uploading your tool to the cloud. Other cloud-container vendors such as may be used instead of Dockerhub.

You’ll also need your tool, the dependencies and know how to install it.

Licenses and warnings

You should have all relevant permission to be able to distribute the tool that you are wrapping. Most open source licenses allow you distribute a compiled or derived version, but may have some restrictions.


It’s important that we consider the versions of our tool when we construct our container. A large factor of this project is reproducibility, and this is only possible if we use the same tools. We highly discourage the use of the latest tag, and instead tag the container with its explicit version.

Note: The same docker tag doesn’t ensure you get the same container each time as a user can upload a new build for the same tag. Ideally you could use the Docker digest however not all container software support this.

Let’s get started!

What are containers

Containers are allow an operating system and software to be virtualised on a different computer (the host). Although containers can be used for a variety of purposes, we use them as a “sandbox” for us to run analysis in. The important aspect for us is that we can guarantee reproducibility with the same container across compute platforms.

In this guide we’ll be creating Docker containers, however other virtualisation software (such as Singularity, uDocker or Shifter) can read this common (OCI compliant) standard.

How do I get a container?

When we look for a container, we want to make sure it’s high quality and is as described. Some services (such as the community supported Biocontainers) produce high quality containers.

Here is a number of criteria we’re looking for in a container:

  • From a reputable source.
  • The Dockerfile is available (gives us trust that there’s not malicious software in there).
  • The versions available are sensible and match the software versions.
    • A container with only the latest tag does not satisfy this criteria.

Here is our recommendation for finding a container given the listed criteria:

  1. Look for a container from the tool provider (maybe through their GitHub).
  2. Look for a container from a reputable source on Dockerhub or
  3. Find a third-party container that meets all the criteria above.

If you are unable to find a container, you might need to build one (and maybe find a tech savvy colleague to help you).

Building a container

We’re going to build a Docker container as at the moment they are the most portable option. Each Docker container has a recipe, called a Dockerfile, it’s a special file that tells Docker how a container is built, and how it should be executed.

Each container should have a base, ie an operating system or even another docker container.

Here are the following types of containers we’ll create:

Basic setup

For all of the following guides, you’ll need to create a folder for your Dockerfile and other dependencies. It’s important you place your dependencies within this directory (or subdirectories) to avoid bloating your build context.

mkdir $tooldir && cd $tooldir
touch Dockerfile

Python module

We’re going to package up a Python module that you can run from the console. For this example we’ll wrap Janis in a container.

Within the, it has a dictionary value within the entry_points kwarg and a value for the console_script key. For example, Janis-assitant looks like this:

    "console_scripts": ["janis=janis_assistant.cli:process_args"],

Given our application is compatible with Python 3.7, we simply pip install our module on top, place this in our Dockerfile:

FROM python:3.7
RUN pip install janis-pipelines
CMD janis
Although janis-pipelines is compatible with the alpine Python 3.7 container, we’ll use the full version, see “The best Docker base image for your Python application” for more information.

We can build this container with the following command which will

  • Use the build context: .
  • Automatically build the file called Dockerfile relative to the build context
  • Give the container the tag: yourname/packagename
docker build -t yourname/janis-pipelines .

We can test the container works by running:

docker run yourname/janis-pipelines janis -v
#--------------------  ------
#janis-core            v0.7.1
#janis-assistant       v0.7.7
#janis-unix            v0.7.0
#janis-bioinformatics  v0.7.1
#--------------------  ------


docker run -it --entrypoint /bin/sh yourname/janis-pipelines
## inside the container
$ janis translate hello wdl 
# translation here
$ exit # exit the container

Python script

We have a single python script that we want to run inside a Docker container. This time we’re going to add our files to the /opt directory (in the container), add that directory to the $PATH variable and then we can simply call your docker container.

We’re going to create a little Python program to write something to the console and then end. Save the following Python program (including the Shebang) into the directory with our Dockerfile.

#!/usr/bin/env python3
print("Hello, World")

We need to make executable by:

chmod +x
We can test this worked by calling our script, by running ./ in our terminal

We’ll now edit our Dockerfile with the following steps:

  • Use a python:3.7 base.
  • Add the file into container.
  • Modify the path to include the script directory (/opt).
  • Use the script an entry-point.
FROM python:3.7.5-alpine
ADD /opt/
ENV PATH="/opt:${PATH}"

We can build this container with the following command:

Don’t forget the full stop . at the end of the docker build command
docker build -t yourname/hello .

You can test that it worked in two ways:

  1. Run the container (using the entrypoint (CMD)
docker run yourname/hello
# Hello, World!
  1. Go into the container, and run the script:
docker run -it --entrypoint /bin/sh yourname/hello
## inside the container
# Hello, World!
$ exit # exit the container

Downloadable Binary with Requirements

This section is still under construction.

This style of container is very similar to the Python script. We’ll ADD our binary (potentially from the web) to /opt/toolname, add this to the path and add an entry point.

You will need to consider which Base OS to use, for now we’ll use alpine as it’s very lightweight, but you might need to consider ubuntu or centos. We’ll assume that your tool mytool runs out of a bin folder.


FROM alpine:latest
RUN mkdir -p /opt/mytool/
ADD /opt/
ENV PATH="/opt/mytool/bin:${PATH}"	# assume ./mytool/bin/
CMD mytool

Makeable Program

This section is under construction, please refer to an example Samtools Docker for more information:


FROM ubuntu:16.04

MAINTAINER Michael Franklin <>

RUN apt-get update -qq \
  && apt-get install -qq bzip2 gcc g++ make zlib1g-dev wget libncurses5-dev liblzma-dev libbz2-dev


  version="${SAMTOOLS_VERSION}" \
  description="Samtools image for use in Workflows"
RUN cd /opt/ \
	&& wget${SAMTOOLS_VERSION}/samtools-${SAMTOOLS_VERSION}.tar.bz2 \
	&& tar -xjf samtools-${SAMTOOLS_VERSION}.tar.bz2 \
	&& rm -rf samtools-${SAMTOOLS_VERSION}.tar.bz2  \
	&& cd samtools-${SAMTOOLS_VERSION}/ \
	&& make && make install

ENV PATH="/opt/samtools-${SAMTOOLS_VERSION}/:${PATH}"

Additional helpful hints

Keeping Dockerfiles separate from source code

You can keep your Dockerfile in a different place to your file (with restrictions) however you must change a few arguments:

  • Your build context (the last argument) should be in a place that can access both the Dockerfile and program files as subdirectories
  • The COPY command in the Dockerfile must be changed to be relative to the build context.
  • You can then supply -f argument to point to the Dockerfile, relative to the build context:

Example: If the python file is stored in a subdirectory called src, and the Dockerfile is stored in a subdirectory called docker_stuff, you could use the following:


COPY src/ /install_dir/


Change into the parent directory of src/ and docker_stuff/

docker build -t yourname/hello -f dockerstuff/Dockerfile .

The less steps your run, the smaller the container

By joining RUN commands together using the && operator, your total containers may be smaller in size.