no such container/image during repack

[jpculp]

Description

When repacking, buildsys attempts to cleanup the buildsys-repack container/image prematurely, printing the following error:

Error response from daemon: No such container: buildsys-repack-aws-fargate-1.4.0-aarch64-45c707cd479e
 
Error response from daemon: No such image: buildsys-repack-aws-fargate-1.4.0-aarch64-45c707cd479e:latest

This error is somewhat hidden, appearing only in build/logs/<ARCH>-<VARIANT>/latest/repack.out, but we should still address it to avoid confusion.

Definition of Done

Suppress the error by iterating over lines in the output, and silencing "No such (container|image): ".

[jpculp]

Be sure to coordinate with @bcressey since the implementation needs to mesh with something he's working on.