no such container/image during repack

## Description

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

```shell
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/-/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): "`.

Start by tracing the buildsys repack cleanup path that produces build/logs/-/latest/repack.out. Inspect how cleanup command output is processed, then run a repack to confirm that "No such container" and "No such image" lines are suppressed while other output remains visible.

Written by the indexing model from the issue text.