This SSH server is based on Ubuntu 20.04 and focussed on connection security and suitable as a SSH tunnel endpoint.
It has the following features and thereby differences to other SSH server images:

• SSH user is (for security purposes) not allowed to authenticate with password
• private key can be created by yourself on your own system
• as many different publickeys as you want/need
• no root login allowed
• image is prepared as a SSH reverse tunnel endpoint

You need a docker host to install.

First create a working directory and it's subdirectory for the public keys on the host:

git clone https://github.com/jotrocken/ssh-server-pubkey.git
cd ssh-server-pubkey

If you already have a RSA keypair, please add it cat <publickeyfile> >> keys/authorized_keys and skip to the next step.

Generate a keypair (public/private) and copy the public key into the right directory (later on this will be a mounted volume).
If you want an automatically establishing SSH connection, don't assign a password for the keyfile!

ssh-keygen -f sshkey -t rsa -b 4096
chmod 600 sshkey
cat sshkey.pub >> keys/authorized_keys

docker run -d \
       -p 4422:22 \
       -v $PWD/keys/:/home/tunnel/.ssh/ \
       --name ssh-server \
       --restart unless-stopped \
       jotrocken/ssh-server-pubkey

docker-compose up -d

Now it's time to try it out! From the docker host, find out the private IP of the container by docker inspect ssh-server, there you find the IP address: "IPAddress": ...
Get a connection: ssh tunnel@[IP] -p 4422 -i sshkey
That's it!

To get a connection from an external machine, move the private keyfile sshkey to that system, adjust your firewall settings on the Docker host (open port 4422), then
ssh tunnel@[dockerhost-public-IP] -p 4422 -i sshkey

Be informed, that your username is tunnel, the password (not usable for SSH login, but for sudo for example) is docker, but you're advised to change it after login: passwd

The second, third, fourth... key can be added by cat <pubkey-filename> >> keys/authorized_keys

I use the following systems at home that provide a HTTP-, SSH- or other login/admin possibility:

• Raspberry Pi
• surveillance camera
• heating control
• smart home tools like plugs to control over internet

Unfortunately my internet connection has no accessible public IP (because of CGNAT), also DynDNS doesn't work because of this. Usually this stuff is connectable to if you're inside our home LAN, but not from outside when I'm not at home.
So I establish a SSH reverse tunnel to a VPS (docker host), there is this container running.

How to do that?

• run new container of this image with minimum one more published port: -p 4401:4401
• or docker-compose -f docker-compose-reversetunnel.yml -d
• create a keypair (see above)
• find out to which port you need to make a connection on that device
• find out the LAN IP of that device and make the IP static in the router settings
• now make a reverse SSH tunnel from that device or any other device in your LAN that "speaks" SSH:

ssh tunnel@[public-IP-of-dockerhost] -p 4422 -i sshkey -fN -R [port-on-home-device]:[LAN-IP-of-home-device]:4401

e.g.:    ssh [email protected] -p 4422 -i sshkey -fN -R 22:192.168.9.112:4401

I recommend to use autossh instead of ssh.

• now you can connect to [public-IP-of-dockerhost]:4401 (think about firewall settings on the docker host!)
• at this container the ports 4401, 4402 and 4403 are ready for this kind of backroute to the dockerhost.

Coming soon...:

• adding fail2ban
• make image size smaller (Alpine?)
• shell script for preparing installation
• optional TOTF authentication (2FA)