Can't scp folder: Host key verification failed

I have a docker container on Server 1 (image node:latest) and want to scp files that are stored in the docker container to a remote Server 2.

I have generated a separate ssh key (no passphrase for server 2 and registered it successfully). Normal login via terminal works:

ssh -i ss-s1-web

The problem is now that within the docker container the following command fails:

$ scp -r -i access/ss-s1-web front root@domain:/home/deploy
Host key verification failed.
lost connection
ERROR: Job failed: exit code 1

What is my misconception about this?

Why does the authentification works from normal terminal but results in a verification fail from within the container?