check for docker before running fn eval

[natasha41575]

fixes #2084

Adds a check for docker before running fn eval.

For example:

$ kpt fn eval simple-function --image gcr.io/kpt-fn/set-namespace:v0.1.3 -- namespace=staging

error: docker must be running to use this command
To install docker, follow the instructions at https://docs.docker.com/get-docker/.

$ kpt fn render basic-pipeline

Package "basic-pipeline":
error: docker must be running to use this command
To install docker, follow the instructions at https://docs.docker.com/get-docker/.

[Shell32-Natsu]

Please add the example output in PR description.

[natasha41575]

Please add the example output in PR description.

Done