Revert "Do not install Docker inside Docker container, while CI testing."

This reverts commit 1cc6045dbaac3025bc46256d8e3be19b1b176d2e,
because Chef seems not to interpret `ENV['DOCKER'].nil?` in its recipes.

This commit refs #257.
Do not install Docker inside Docker container,  while CI testing.

Prerequisite for installing Docker is: update Linux kernel,
but updating Linux kernel is not automated.

Fow workaround to this problem,
do not install Docker inside Docker container, while CI testing.

This commit fixes #257.
Merge branch 'id/257' into dev