refactor(docker): remove placeholder image names

[roschaefer]

Motivation

The image in a docker-compose.yml is not required. If you want to build the docker image from the Dockerfile you can simply add a build context.

How to test

1. docker image rm all the dreammall images
2. docker compose up
3. All images get rebuilt

[Tirokk]

There was a reason why I added code like this to the docker-compose files of ocelot.social and I guess this was a template here.
Before removing this please lets talk.
May I haven't understood something at that time.

Unfortunately I cannot remember my reason at the moment.

[mahula]

There was a reason why I added code like this to the docker-compose files of ocelot.social and I guess this was a template here.

Right, this has just been copied to this project.

Unfortunately I cannot remember my reason at the moment.

Original reason in earlier projects was the busFaktor e.V. running a DockerHub instance as a registered OSS organisation representing all Ocelot.social related docker images.
This does not apply to this project.
This might make the comment in the file obsolete, but we should keep the image parameter to control the image naming for all the contexts they are configured for in the different docker yaml files.
Further ideas regarding the control of docker-related things might be disscussed further in issues related to #1076.