update the apel/ssm container

[rptaylor]

Thanks to apel/ssm#185 it looks like an official ssmsend container is now available here built from this Dockerfile.

It could be nice to use that instead of building our own. That being said the Dockerfile has not been updated for 6 years and seems to include dev stuff. Also it does not update the centos7 container to the latest packages (I just tried based on the kapel Dockerfile and the official centos7 image still had 51 updates to apply). Also it is pip based rather than RPM based. Might need to update/test/tweak some things.

[rptaylor]

Apparently the latest centos7 image was last updated 2 years ago:

$ buildah images
REPOSITORY                               TAG      IMAGE ID       CREATED          SIZE
registry.hub.docker.com/library/centos   7        eeb6ee3f44bd   2 years ago      212 MB

[rptaylor]

Or if we continue to build our own container, there should be new SSM packages for EL9 available soon we should start to use.

[rptaylor]

EL9 packages available now:
https://github.com/apel/ssm/releases/tag/3.4.0-1

[rptaylor]

Updated in #72 for now but it might still be a good idea to use the upstream container.