The Alpine ARM docker image is used for cross compilation. There was a
breaking change in Alpine 3.13 - it uses a new MUSL that has 64 bit
time_t even on 32 bit platforms. Since the official support for Alpine
ARM was not announced yet, it was decided that we will support it only
on Alpine >= 3.13 instead of having to build and distribute two different
versions of runtime.

This change updates the docker image used to build Alpine ARM runtime to
a new version that uses Alpine 3.13 rootfs. It is also using Ubuntu
16.04 instead of 18.04 as a host to unify it with the one we use to
build for Alpine ARM64.