"The ECDSA host key has changed" error with some docker images

Note: I'm trying to learn how to use the Pipelines feature so I might be missing something, in such case sorry for that but I can't find any information about this issue.

I successfully run some pipelines using the default docker image or the suggested PHP one, but if I try to use both "ubuntu" or "debian" it fails to connect using ssh:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@       WARNING: POSSIBLE DNS SPOOFING DETECTED!          @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
The ECDSA host key for ********** has changed,
and the key for the corresponding IP address ***.***.***.***
is unknown. This could either mean that
DNS SPOOFING is happening or the IP address for the host
and its host key have changed at the same time.
...

Note: running cat ~/.ssh/known_hosts shows GitHub, Bitbucket and my custom host.

The same YML file (changing the docker image back to "php" or the default one) works fine... what am I missing? I'm doing these tests to understand the platform and start building custom images with my own tools already built-in.

Here is the pipeline file:

#!yml
image: debian:stable-slim

pipelines:
  default:
    - step:
        script:
          - apt-get update && apt-get install -y openssh-client
          - scp -r $USER@$HOST:$PATH .
          - ls -hga test

Thank you for your time.