feat: allow overriding docker executable

[afbjorklund]

So that you can run with DOCKER=nerdctl

Closes #36

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Note, if testing with nerdctl.lima also need:

TMPDIR=~/.local/tmp

[nalgeon]

Hey Anders, thank you for the PR! The build is failing because of unrelated problems — please merge the latest changes from the main branch to fix them.

[nalgeon]

Thank you!

[afbjorklund]

I should have mentioned that the need for overriding TMPDIR is that the default /tmp is not mounted on the VM.

When you are running with docker or podman-remote, there is a network mount of the files from the host system. But when running with lima nerdctl or lima podman, there is only a local mount of the files in the guest system.

So in order to mount something from the host, it needs to have been configured already - and only HOME is shared.