Change Dockerfile python package to python3

[jairusjoer]

As of version 3.12, the python package will no longer be supported in favor of explicitly chosen packages python2 or python3.

For more information regarding this change, refer to the following commit:

• https://git.alpinelinux.org/aports/commit/?id=5ad0ec7d

or discussion on the docker-alpine repository:

• Errors when installing python packages using alpine:3.11 tag alpinelinux/docker-alpine#83 (comment)

[amark]

@tcurdt I assume this won't break anything? Will merge if I don't hear a reply.

Thanks @jairusjoer !

[tcurdt]

Looking good!