kas: Adding SSH key using SSH_PRIVATE_KEY fails

Environment

Docker image: kasproject/kas:2.2 SSH private key in PEM RSA format

Expected behavior

running SSH_PRIVATE_KEY=/path/to/ssh/key kas build /mnt/kas-foo.yml in the docker container should add the SSH key to the ssh-agent using ssh-add.

Actual behavior

builder@e76a2ef8b3c4:/tmp$ SSH_PRIVATE_KEY=/path/to/ssh/key kas build /mnt/kas-foo.yml 
2020-09-09 11:08:45 - INFO     - kas 2.2 started
2020-09-09 11:08:45 - INFO     - /mnt$ git rev-parse --show-toplevel
2020-09-09 11:08:45 - INFO     - adding SSH key
2020-09-09 11:08:45 - ERROR    - failed to add ssh key: b'Error loading key "(stdin)": invalid format\n'

Additional information

Manually running ssh-agent -s and ssh-add /path/to/ssh/key works:

builder@e76a2ef8b3c4:/tmp$ ssh-agent -s
SSH_AUTH_SOCK=/tmp/ssh-Ds4LFCbiHB34/agent.122; export SSH_AUTH_SOCK;
SSH_AGENT_PID=123; export SSH_AGENT_PID;
echo Agent pid 123;
builder@e76a2ef8b3c4:/tmp$ ssh-add /path/to/ssh/key
Identity added: /path/to/ssh/key (/path/to/ssh/key)