Add a autoconf variable to allow the specification of a "small" docker image

[jodh-intel]

Lots of files currently hardcode the docker image busybox as it is a very small but useful docker image [1].

Rather than hard-coding it everywhere we should define it in configure.ac to allow all files in the repo to make use of the variable.

We could also provide a --with-small-docker-image-name= option to configure.ac to allow the default to be changed. This would be useful to shake out any bugs we have where a busybox environment is assumed.

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The current list:

• data/make-bundle-dir.sh
• tests/integration/docker/build.bats
• tests/integration/docker/commit.bats
• tests/integration/docker/create.bats
• tests/integration/docker/export.bats
• tests/integration/docker/info.bats
• tests/integration/docker/inspect.bats
• tests/integration/docker/kill.bats
• tests/integration/docker/load.bats
• tests/integration/docker/port.bats
• tests/integration/docker/restart.bats
• tests/integration/docker/run.bats
• tests/integration/docker/tag.bats
• tests/integration/docker/user.bats
• tests/integration/docker/volume.bats
• tests/metrics/workload_time/cor_create_time.sh.in