Make the --pull behaviour being configurable over env var

[jcchavezs]

Description

Given this great issue #1498 I always wondered if the default could be configured over an env var for a user.

Describe the results you expected:

$DOCKER_PULL_IMAGES=missing|never|always

Ping @yshen

[thaJeztah]

Discussing this with other maintainers, and we currently don't plan to add environment variables for specific options/flags, but you can probably create a shell alias if you want to set this option by default.

If you're using Docker Compose, you can also set the pull-policy in a compose-file (which may help for such scenarios)