Add swh-utils image to help build init-container image

We have currently some init-container image which requires extra dependencies to do their job. They currently pull the debian:bullseye and installs those prior to run the script.

Building this image (and declaring it there) will simplify and accelerate the process of initialization.

The image is slighly bigger than the default (+26MB).

$ docker image ls swh-utils:latest
REPOSITORY   TAG       IMAGE ID       CREATED         SIZE
swh-utils    latest    18fc5c089314   6 seconds ago   150MB
$ docker image ls debian:bullseye
REPOSITORY   TAG        IMAGE ID       CREATED       SIZE
debian       bullseye   873181a41b99   2 weeks ago   124MB

Refs. sysadm-environment#5179 (closed)