* Use skopeo to download container images
Separate container download from image build. This will allow to share
the downloaded images between multiple builds.
We won't store the Supervisor container with the version tag, just with
the latest tag. This allows to simplify the procedure a bit. It seems
there is no downside to this approach.
* Use official Docker in Docker images to build data partition
Instead of building our own Debian based image let's use the official
Docker in Docker image. This avoids building an image for the hassio
data partition and speeds up build as well.
This calls mount commands using sudo to mount the data partition as part
of the buildroot build now. This is not much different from before as
mount has been called as root inside the container, essentially equates
to the same "isolation" level.
* Use image digest as part of the file name
The landing page has no version information in the tag. To avoid
potentially source caching issues, use the digest as part of the file
name.
Virtual Disk images are often used on Windows and/or Mac platforms where
xz is not a widely known file ending and also not supported by dafault.
Use zip which is much better known.
Keep using xz for boards since those are not meant to be extracted by
users but directly used in Etcher. Also keep using xz for qcow2, since
qcow2 is mostly used on Linux platforms where xz is available by default
and zip usually needs an extra package.
