As mentioned in #7397, switching to a debian base should help with multi-arch work to save time on compiling. This is unashamedly based on #6373, but without the extra functionality. Switch python version back to generic 3.7 to always pull the latest. Essentially, keeping this as small as possible. The image is bigger though unfortunately.
Alpine Linux 3.8 is still supported, but it seems like
it's quite outdated now.
While Python should be the same on both, all other libraries, etc.,
are much newer in Alpine 3.9 and 3.10.
Signed-off-by: Slavi Pantaleev <slavi@devture.com>
There are two reasons this is a good thing:
* first, it means that you don't end up with stuff kicking around your working
copy ending up in the build image by mistake (which can upset the pip
install process)
* second: it means that the docker image cache is more effective, and we can
reuse docker images when iterating on the docker stuff.
