docker context and DOCKER_HOST env support

[h0tw1r3]

Adds support for parsing the docker context or DOCKER_HOST environment variable to determine the hostname used to connect to the instance.

[CLAassistant]

All committers have signed the CLA.

[h0tw1r3]

Until this is merged, here's my painless hack.

Drop the following code into a rake file (ie. rakelib/litmus.rake). When you run provision tasks, it will automatically patch the generated inventory file for remote docker support.

# Patch docker targets in inventory to support DOCKER_HOST environment variable
task :patch_docker_inventory do
  require 'yaml'
  require 'uri'
  hostname = if ENV.key? 'DOCKER_HOST'
               URI.parse(ENV['DOCKER_HOST']).host || ENV['DOCKER_HOST']
             else
               'localhost'
             end
  if hostname != 'localhost'
    inventory_fn = "#{__dir__}/../spec/fixtures/litmus_inventory.yaml"
    data = YAML.load_file(inventory_fn)
    data['groups'].map do |group|
      if group['targets'].size > 0
        group['targets'].map do |target|
          target['uri'].gsub!('localhost', hostname) if target['facts']['provisioner'] == 'docker' && target['uri'] =~ %r{^localhost:}
        end
      end
    end
    File.open(inventory_fn, 'w') { |f| f.write(data.to_yaml) }
  end
end

Rake::Task['litmus:provision'].enhance do
  Rake::Task['patch_docker_inventory'].execute
end
Rake::Task['litmus:provision_list'].enhance do
  Rake::Task['patch_docker_inventory'].execute
end

[jordanbreen28]

hey @h0tw1r3 thanks for this, its great!
Just a couple of requests before we can merge..

Could you please:

1. Add a spec test for this new functionality
2. Add a section in the README docker section explaining how to use this
3. rebase off main

Thanks again. 🚀

[h0tw1r3]

Could you please:

1. Add a spec test for this new functionality

@jordanbreen28 Can you point me to the existing docker spec tests?

[h0tw1r3]

Standard use cases are a docker CI runner using a remote docker to avoid the pitfalls of docker-in-docker, or a workstation which must to use a remote docker service (ie. Windows in AWS workspaces, security requirement, etc).

The "DOCKER_HOST" way, is universally supported by all docker versions.

export DOCKER_HOST=docker0.dev.internal:2376
pdk bundle exec rake 'litmus:provision[docker, litmusimage/ubuntu:22.04]'
pdk bundle exec rake litmus:install_agent
pdk bundle exec rake litmus:acceptance:parallel

The modern way, is to use contexts.

# create remote context
docker context create remote0 --description 'remote docker0' --docker "host=tcp://docker0.dev.internal:2376"

# run acceptance tests on remote docker
docker use remote0
pdk bundle ...

# run acceptance tests on local docker
docker use default
pdk bundle ...