docker_helper.rb

# frozen_string_literal: true

# DockerHelper provides methods to work with Docker containers.
module DockerHelper
  extend self

  def running_in_docker?
    !!(File.read("/proc/1/cgroup") =~ %r[^\d+:\w+:/docker/]) # !! => true/false
  rescue Errno::ENOENT
    false
  end
end

docker_helper_spec.rb

# frozen_string_literal: true

require "spec_helper"

RSpec.describe DockerHelper do
  let(:cgroup_docker) do
    <<~CGROUP
    3:cpuacct:/docker/deadbeef
    2:cpu:/docker/acecafe8
    1:cpuset:/docker/104decaf
    CGROUP
  end

  let(:cgroup_linux) do
    <<~CGROUP
    3:cpu,cpuacct:/init.scope
    2:devices:/init.scope
    1:name=systemd:/init.scope
    CGROUP
  end

  it "detects running on Docker" do
    # /proc/1/cgroup contains Docker strings
    allow(File).to receive(:read).and_return(cgroup_docker)
    expect(DockerHelper.running_in_docker?).to eq(true)
  end

  it "detects not running on Docker (Linux)" do
    # /proc/1/cgroup contains non-Docker strings
    allow(File).to receive(:read).and_return(cgroup_linux)
    expect(DockerHelper.running_in_docker?).to eq(false)
  end

  it "detects not running on Docker (macOS)" do
    # /proc/1/cgroup doesn't exist
    allow(File).to receive(:read).and_raise(Errno::ENOENT)
    expect(DockerHelper.running_in_docker?).to eq(false)
  end
end