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Rum Runner is a Rake-based utility for building multi-stage Dockerfiles.

Users can pair a multi-stage Dockerfile with a Rumfile that uses a Rake-like DSL to customize each stage's build options and dependencies.

The rum executable allows users to easily invoke builds, shell-into specific stages for debugging, and export artifacts from built containers.

Rum Runner has the following features:

  • Fully compatible with Rake
  • Rake-like DSL/CLI that enable simple annotation and execution of builds
  • Rumfiles are completely defined in standard Ruby syntax, like Rakefiles
  • Users can chain Docker build stages with prerequisites
  • Artifacts can be exported from stages
  • Shell tasks are automatically provided for every stage
  • Stage, artifact, and shell, steps can be customized


This project was born from using Makefiles to drive multi-stage builds. For the most part this worked really well, but it became a bit of an ordeal to write for more complex projects. This tool is an attempt to recreate that general technique with minimal annotation and limited assumptions.

View the docs on


gem install rumrunner


If you have a multi-stage Dockerfile in your project and are unsure where to begin, use the rum init helper to create a template Rumfile for your project:

gem install rumrunner
rum init > Rumfile
rum --tasks

The init command will parse a Dockerfile in the current directory and output a simple Rumfile with each stage and its dependencies declared.


Imagine a simple multi-stage Dockerfile:

FROM ruby AS build
# Run build steps here...

FROM ruby AS test
# Run test steps here...

FROM ruby AS deploy
# Run deploy steps here...

Create Rumfile and describe your build:

rum :image_name do
  tag "1.2.3"

  stage :build
  stage :test => :build
  stage :deploy => :test

  # rum build  => docker build --target build  --tag image_name:1.2.3-build .
  # rum test   => docker build --target test   --tag image_name:1.2.3-test .
  # rum deploy => docker build --target deploy --tag image_name:1.2.3-deploy .

Run rum --tasks to view the installed tasks:

rum build                # Build `build` stage
rum clean                # Remove any temporary images and products
rum clean:build          # Remove any temporary images and products through `build` stage
rum clean:deploy         # Remove any temporary images and products through `deploy` stage
rum clean:test           # Remove any temporary images and products through `test` stage
rum clobber              # Remove any generated files
rum deploy               # Build `deploy` stage
rum shell:build[shell]   # Shell into `build` stage
rum shell:deploy[shell]  # Shell into `deploy` stage
rum shell:test[shell]    # Shell into `test` stage
rum test                 # Build `test` stage

Customize Shells

By default, all stages have a :shell task that can be invoked to build and shell into a container for a stage. By default the container is run as an ephemeral container (--rm) in interactive with TTY allocated and a bash shell open.

Customize the shell for a stage with the shell method:

rum :image_name do
  stage :dev

  shell :dev do
    entrypoint "/bin/zsh"
    rm         false
    volume     "#{Dir.pwd}:/var/task/"

  # rum dev => docker run --entrypoint /bin/zsh --volume $PWD:/var/task/ ...

Customize Stages

Stages can be customized with blocks:

rum :image_name do
  tag "1.2.3"

  stage :build

  stage :test => :build

  stage :deploy => :test do
    build_arg :AWS_ACCESS_KEY_ID
    build_arg :AWS_SECRET_ACCESS_KEY
    build_arg :AWS_DEFAULT_REGION => "us-east-1"
    label     :Fizz

Methods invoked inside the stage block are interpreted as options for the eventual docker build command.

Export Artifacts

Use the artifact method to specify an artifact to be exported from the image.

rum :image_name do
  stage :build

  artifact "" => :build

By default the container simply cats the file from the container to the local file system, but more complex exports can be defined:

rum :image_name do
  stage :build

  artifact "" => :build do
    workdir "/var/task/"
    cmd     %w[zip -r - .]

Methods invoked inside the artifact block are interpreted as options for the eventual docker run command.

Default Task

Every rum declaration has a default task associated with it so that simply executing rum on the command line does something.

In the most simple case, the default task simply builds the image:

rum :image_name
# rum => docker build --tag image_name .

Use the default method inside the main block to set a default task or tasks:

rum :image_name do
  stage :build
  stage :plan => :build

  artifact "" => :build

  default ["", :plan]
# rum => docker build --target build ...
#        docker run ... >
#        docker build --target plan ...

Shared ENV variables

The env method can be invoked in the rum block to declare a value that will be passed to all stages/artifacts/shells. For stages, the value will be passed using the --build-arg option; for artifacts and shells, the --env option.

rum :image_name do
  env :FIZZ => :BUZZ

  stage :build

  # rum build => docker build --build-arg FIZZ=BUZZ ...


Run a stage task to build the image up to that stage and cache the image digest.

Run with the :shell suffix to build the image and then shell into an instance of the image running as a temporary container.

The default shell is /bin/sh, but this can be overridden at runtime with the task arg, eg. rum build:shell[/bin/bash]

Build vs. Run

At the core, every directive within the rum block will eventually be interpreted as either a docker build or a docker run command. The type of directive is simply a way of specifying defaults for the command.

If you simply wish to define a named task that executes a build or a run, you can use the build or run directives:

rum :image_name do
  env :JAZZ => "fuzz"

  build :fizz do
    tag  "image_name"
    path "."

  run :buzz do
    rm    true
    image "image_name"
    cmd   %w[echo hello]

  # rum fizz => docker build --build-arg JAZZ=fuzz --tag image_name .
  # rum buzz => docker run --rm --env JAZZ=fuzz image_name echo hello

Note that the build/run commands will still import any shared ENV values defined above.

If this is undesirable, use the clear_options method inside your block to clear ALL the default options:

rum :image_name do

  env :JAZZ => "fuzz"

  run :buzz do
    image "image_name"
    cmd   %w[echo hello]

  # rum buzz => docker run image_name echo hello


The methods inside blocks for build, run, stage, artifact, and shell tasks are dynamically handled. Any option you might pass to the docker run or docker build command can be used.

Simply drop any leading -s from the option and convert to snake-case.


--build-arg becomes build_arg

--env-file becomes env_file.

Task Naming Convention

As of v0.3, rum runner uses a "verb-first" naming convention (eg. clean:stage) for tasks.

To revert to the previous convention of "stage-first" (eg. stage:clean) use the environmental variable RUM_TASK_NAMES:

export RUM_TASK_NAMES=STAGE_FIRST  # => rum stage:clean
export RUM_TASK_NAMES=VERB_FIRST   # => rum clean:stage (default)

Image Naming Convention

The name of the images are taken from the first argument to the main block and appended with the name of the stage.

In the above example, built images would build be named:

  • image_name:1.2.3-build
  • image_name:1.2.3-test
  • image_name:1.2.3-deploy

The first argument to the main block can be any Docker image reference:

rum :"registry:5000/username/image" do

Dockerfile Location

Images built use the current working directory as the default path to the Dockerfile, but this can be modified:

rum :image_name => "some/dockerfile/dir" do
  # ...

The default Dockerfile path can also be set using the RUM_PATH environmental variable.

Docker Image Digest Location

Images build with the stage task have their digests cached for easy lookup.

The default location for the digests is in .docker, but that can be modified:

rum :image_name => [".", "tmp"] do
  # ...

Note that in this case you must also explicitly define the Dockerfile path.

The default digest path can also be set using the RUM_HOME environmental variable.

Integrate with Rake

It isn't strictly necessary to include a Rumfile in your project. Rum Runner can be included in any Rakefile and run with the rake command:

# ./Rakefile

require "rumrunner"

namespace :rum do
  rum :image_name do
    stage :build
    stage :test => :build
$ rake --tasks

rake rum:build # ...
rake rum:test  # ...