Set SecurityContext for connection pooler

[addreas]

Set uid/gid based on whats burned into the registry.opensource.zalan.do/acid/pgbouncer image.

[FxKu]

I don't see where we set those IDs in the pooler repo. Are these default values? We use:

RUN \
    addgroup -S pgbouncer && \
    adduser -S pgbouncer && \

USER pgbouncer:pgbouncer

[addreas]

I think I had a look in /etc/passwd and /etc/group

[FxKu]

Closing because the suggested changes have been reflected by #2255
Thanks @addreas for raising this topic.