Support to login docker in GitPod automatically

Created by: LinuxSuRen

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Users don't need to log in to their docker hub account manually.

How to?

You can put these env into your GitPod account: DOCKER_USER and DOCKER_PASSWD

Which issue(s) this PR fixes:

None.

Special notes for reviewers:

None

Does this PR introduced a user-facing change?

None

Additional documentation, usage docs, etc.:

None