$ brew cask install docker-toolbox

Failure 言われてる…

$ brew cask install docker-toolbox
Updating Homebrew...

<...>

==> Purging files for version 6.0.4,128413 of Cask virtualbox
Error: Failure while executing; `/usr/bin/sudo -E -- env LOGNAME=_____ USER=_____ USERNAME=_____ /usr/sbin/installer -pkg /usr/local/Caskroom/virtualbox/6.0.4,128413/VirtualBox.pkg -target /` exited with 1. Here's the output:
installer: Package name is Oracle VM VirtualBox
installer: Installing at base path /
installer: The install failed (The Installer encountered an error that caused the installation to fail. Contact the software manufacturer for assistance.)

Follow the instructions here:
  https://github.com/Homebrew/homebrew-cask#reporting-bugs
/usr/local/Homebrew/Library/Homebrew/system_command.rb:107:in `assert_success'
/usr/local/Homebrew/Library/Homebrew/system_command.rb:50:in `run!'
/usr/local/Homebrew/Library/Homebrew/system_command.rb:27:in `run'
/usr/local/Homebrew/Library/Homebrew/system_command.rb:31:in `run!'
/usr/local/Homebrew/Library/Homebrew/cask/artifact/pkg.rb:57:in `block in run_installer'
/usr/local/Homebrew/Library/Homebrew/cask/artifact/pkg.rb:63:in `with_choices_file'
/usr/local/Homebrew/Library/Homebrew/cask/artifact/pkg.rb:50:in `run_installer'
/usr/local/Homebrew/Library/Homebrew/cask/artifact/pkg.rb:32:in `install_phase'
/usr/local/Homebrew/Library/Homebrew/cask/installer.rb:211:in `block in install_artifacts'
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/set.rb:674:in `each'
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/set.rb:674:in `each'
/usr/local/Homebrew/Library/Homebrew/cask/installer.rb:202:in `install_artifacts'
/usr/local/Homebrew/Library/Homebrew/cask/installer.rb:101:in `install'
/usr/local/Homebrew/Library/Homebrew/cask/installer.rb:342:in `block in cask_dependencies'
/usr/local/Homebrew/Library/Homebrew/cask/installer.rb:335:in `each'
/usr/local/Homebrew/Library/Homebrew/cask/installer.rb:335:in `cask_dependencies'
/usr/local/Homebrew/Library/Homebrew/cask/installer.rb:248:in `satisfy_dependencies'
/usr/local/Homebrew/Library/Homebrew/cask/installer.rb:61:in `fetch'
/usr/local/Homebrew/Library/Homebrew/cask/installer.rb:92:in `install'
/usr/local/Homebrew/Library/Homebrew/cask/cmd/install.rb:21:in `block in run'
/usr/local/Homebrew/Library/Homebrew/cask/cmd/install.rb:14:in `each'
/usr/local/Homebrew/Library/Homebrew/cask/cmd/install.rb:14:in `run'
/usr/local/Homebrew/Library/Homebrew/cask/cmd/abstract_command.rb:34:in `run'
/usr/local/Homebrew/Library/Homebrew/cask/cmd.rb:90:in `run_command'
/usr/local/Homebrew/Library/Homebrew/cask/cmd.rb:156:in `run'
/usr/local/Homebrew/Library/Homebrew/cask/cmd.rb:121:in `run'
/usr/local/Homebrew/Library/Homebrew/cmd/cask.rb:7:in `cask'
/usr/local/Homebrew/Library/Homebrew/brew.rb:100:in `<main>'

解決方法

システム環境設定 -> セキュリティとプライバシー

赤枠部分に「Oracleのソフトウェアを許可しますか？」
みたいなことが書かれているので、許可します。

$ brew cask install docker-toolbox

<...>

🍺  docker-toolbox was successfully installed!