Move dea advertisement subscription to stager_pool

[#75217152]

Signed-off-by: Peter Parente <pparent@us.ibm.com>
Remove unused method

[#75217152]

Signed-off-by: Peter Parente <pparent@us.ibm.com>
Add `docker_image` to app table, model, controller

API spec is updated with experimental support for docker images

[#75217152]

Signed-off-by: Peter Parente <pparent@us.ibm.com>
Avoid nil start commands when droplet is missing

Signed-off-by: Peter Parente <pparent@us.ibm.com>