builder: Appropriately initialize the Cmd in runconfig.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
builder: integration-cli test for the previous commit.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Merge pull request #8179 from erikh/parser_fix_cmd_null

Parser fix cmd null