Write imageid to .dobi dir on pull action

[lhuhn-wish]

On a pull action, dobi wasn't filling in the imageid field in the .dobi directory, causing a later build action to fail to use the cached version on the check:

if image.ID != record.ImageID || record.Info.ModTime().Before(mtime) { t.logger().Debug("Image record older than context") return true, nil } This change fixes it so dobi writes the imageid on pulls.

[CLAassistant]

All committers have signed the CLA.

[lhuhn-wish]

There now the CLAassistant is happy.

[lhuhn-wish]

Bump. This one is fairly straightforward, I hope. Before the patch the files in .dobi look like this after a pull:

imageid: "" lastpull: 2020-08-04T11:55:49.771412103-05:00

and after the patch they look like:

imageid: sha256:ea2320b68c0ad296a1665a0dd061ca17e1d213792fa6db7446d79a1a35a6aee4 lastpull: 2020-08-04T11:56:44.365497112-05:00

[dnephin]

LGTM, thanks!