add a Dockerfile to build the disk image

[tesuji]

I split to original files to 2 more files to enable docker cache the debootstrap step (around 120s up to 200s).

Notes: git show main...<this branch> could highlight moved lines between files while GitHub UI could not.

[Kyle-Kyle]

thanks for the PR.
I think creating the disk image in a docker container is a good idea. But splitting the create_image.sh script into multiple ones is not necessary, especially when some of them may potentially cause issues.
For example, if a user accidentally executes the configure.sh file, their machine may get corrupted. Even though it is not a scenario likely to happen, I'd rather not have this risk in the repo.

[tesuji]

That's a valid concern. I will add a magic env var and only execute configure.sh if that var presents.