Build the integration-test images bottom-to-top

`IntegrationTestDockerImages` depended on the packaging job for both its debian
packages and the published generic daemon image. Each step now packages only
the debs its image installs, out of the app-build binaries, and builds the image
locally with `--load-only`, so the job depends on nothing but the apps build.
The daemon step builds the generic image itself before the profiled one that is
`FROM` it.

The deb-then-image sequence is now shared with `AutoHardforkTest`:
`buildkite/scripts/docker/build-from-local-debs.sh` does the per-image work and
`buildkite/src/Command/DockerFromLocalDebs.dhall` renders the invocation,
replacing the bespoke `build-auto-hardfork-image.sh`.

Step keys and labels are unchanged, so the integration tests' dependencies keep
resolving.
