Running ssh-agent within docker on jenkins doesnt work - docker

Question

I am trying to use a container within my jenkins pipeline, however I cant get ssh-agent to work inside it. I am on v1.19 of the plugin, when I run the below code I get
Host key verification failed. fatal: Could not read from remote
repository.
Please make sure you have the correct access rights and the repository
exists.
However if I run the code from outside the image it works perfect, proving that the user has the correct permissions.
node('nodeName'){
cleanWs()
ws("short"){
withDockerRegistry([credentialsId: 'token', url: "https://private.repo.com"]) {
docker.image("img:1.0.0").inside("-u root:root --network=host") {
sshagent(credentials: ["bitbucket_token"]) {
sh "mkdir ~/.ssh"
sh 'ssh-keyscan bitbucket.company.com >> ~/.ssh/known_hosts'
sh 'git clone ssh://git#bitbucket.company.com:PORT/repo.git'
}
}
}
}
}
Here is the output:
[Pipeline] sshagent
[ssh-agent] Using credentials jenkins (bitbucket_token)
[ssh-agent] Looking for ssh-agent implementation...
[ssh-agent] Exec ssh-agent (binary ssh-agent on a remote machine)
$ docker exec abcdef123456 ssh-agent
SSH_AUTH_SOCK=/tmp/ssh-qwertyu/agent.15
SSH_AGENT_PID=22
Running ssh-add (command line suppressed)
Identity added: /home/jenkins/short#tmp/private_key_8675309.key (/home/jenkins/short#tmp/private_key_8675309.key)
[ssh-agent] Started.
[Pipeline] {
[Pipeline] sh
+ mkdir /root/.ssh
[Pipeline] sh
+ ssh-keyscan bitbucket.company.com
# bitbucket.company.com:22 SSH-2.0-OpenSSH_6.6.1
# bitbucket.company.com:22 SSH-2.0-OpenSSH_6.6.1
# bitbucket.company.com:22 SSH-2.0-OpenSSH_6.6.1
[Pipeline] sh
+ git clone ssh://git#bitbucket.company.com:PORT/repo.git
Cloning into 'repo'...
Host key verification failed.
fatal: Could not read from remote repository.
Please make sure you have the correct access rights
and the repository exists.
[Pipeline] }
$ docker exec --env ******** --env ******** abcdef123456 ssh-agent -k
unset SSH_AUTH_SOCK;
unset SSH_AGENT_PID;
echo Agent pid 22 killed;
[ssh-agent] Stopped.
[Pipeline] // sshagent
Im completely stumped by this