Missing userland binaries for PostGIS 2.1

Hi all-

It looks like the apt.postgresql.org repo was recently updated to have
PostGIS 2.2 packages. I build a relatively popular Docker image for
PostGIS, so this was great to see since it's something users have been
asking to have for some time (see
https://github.com/appropriate/docker-postgis)

However, it looks like in the process of updating the repo, version 2.1 of
the "postgis" package was removed. This means that I can no longer build an
image for PostGIS 2.1 on older Postgres versions that includes the PostGIS
userland binaries.

I understand the difficulty of providing both a 2.2 package and a 2.1
package in the same repo since there is no version number in the package
name (e.g. "postgis-2.1" and "postgis-2.2"), so I'm wondering how much of a
loss it is to no longer have the userland binaries. I personally don't use
those binaries, so I can't be sure of the impact of simply removing them
from the Docker image.

-md