Update image locations in CommonTasks and README files

[TimHess]

As pointed out here, we're not publishing images to hub.docker.com anymore, and any documentation of our docker images needs to be updated accordingly

[bart-vmware]

Until this is implemented, SteeltoeOSS/Dockerfiles#36 (comment) describes how to change existing commands.

The docker commands change a little bit, but not by much (eg: steeltoe.azurecr.io/spring-boot-admin:3.3.3 compared with docker.io/steeltoeoss/spring-boot-admin:3.3.3, where docker.io/ was generally implicit).

[bart-vmware]

We may want to add --pull-always to the docker commands, to ensure a newer version is fetched when available.