### What
a new Dockerfile to install Core from the "testing" pool
an update to the Makefile specifically for this case

### Why
adding support for the "testing" pool to the Dockerfile would make
the file more confusing to read, so I decided to use a separate Dockerfile

### Testing
I will test this manually shortly

### Issue addressed by this PR
stellar/ops#3124

### What
build horizon docker image from core "stable" and horizon "stable"

### Why
this is needed in order to support the new workflow described in the
ticket

### Testing
this has been tested and works with apt "testing"

### Issue addressed by this PR
stellar/ops#3124