use of 'apt' in scripting in our dockerfiles provokes warnings

[Peter Maydell]

While processing a recent pullreq which updated the dockerfile
dependencies and thus provoked a rebuild, I noticed that we
use the 'apt' command in ways that provoke a warning:

#7 0.789 WARNING: apt does not have a stable CLI interface. Use with
caution in scripts.

The apt(8) manpage says:

SCRIPT USAGE AND DIFFERENCES FROM OTHER APT TOOLS
       The apt(8) commandline is designed as an end-user tool and it may
       change behavior between versions. While it tries not to break backward
       compatibility this is not guaranteed either if a change seems
       beneficial for interactive use.

       All features of apt(8) are available in dedicated APT tools like apt-
       get(8) and apt-cache(8) as well.  apt(8) just changes the default value
       of some options (see apt.conf(5) and specifically the Binary scope). So
       you should prefer using these commands (potentially with some
       additional options enabled) in your scripts as they keep backward
       compatibility as much as possible.

That suggests that we should probably be using apt-get instead
of apt in our dockerfiles...

Also, any idea what this error is about?

#4 importing cache manifest from registry.gitlab.com/qemu-project/qemu/qemu...
#4       digest:
sha256:815fb44b573ac7520d148d9b2510c00f31846ca6fa55127d322bc8db3c5d0ec0
#4         name: "importing cache manifest from
registry.gitlab.com/qemu-project/qemu/qemu/debian-alpha-cross:latest"
#4      started: 2020-09-29 22:57:54.371490684 +0000 UTC
#4    completed: 2020-09-29 22:57:55.064468936 +0000 UTC
#4     duration: 692.978252ms
#4        error: "invalid build cache from
{MediaType:application/vnd.docker.distribution.manifest.v2+json
Digest:sha256:a1e8a5830bb19b7cddda64872c5d71a0337d4b98bed30fd7684d20467adcd289
Size:1161 URLs:[] Annotations:map[] Platform:<nil>}"

It didn't seemt to have an adverse effect...

thanks
-- PMM