Skip sleep and warning of 'command_exists docker' when relevant flag is set

[Tashows]

Closes #183

- What I did Added the ability to skip sleep related to command_exists docker check.

- How I did it Added a relevant flag that skips block of code when it is present. Previous/Default functionality is not affected.

- How to verify it Run SKIP_DOCKER_EXISTS_WARN=true ./install.sh vs ./install.sh to verify relevant warning and sleep time do not appear/occur.

- Description for the changelog

Skip sleep and warning of 'command_exists docker' when SKIP_DOCKER_EXISTS_WARN is set

- A picture of a cute animal (not mandatory but encouraged)