docs: Change docker example to be more robust.

[Potherca]

Q	A
Bug fix?	no
New feature?	no
Fixed tickets	no

The docker example uses pwd, but it is not wrapped in quotes ".

If the path returned by pwd contains a space character, docker will have an error:

docker: invalid reference format.

For people less familiar with docker, it can be quite unclear what is going wrong.

This MR fixes that issue by wrapping the call in quotes, so the path is passed to docker properly.

[Jibbarth]

Thank you for this @Potherca 👍