keter is an application server. You create a bundle (a gzip'ed tar file ending in the extension .keter) from your Yesod Stack project by running the command yesod keter. To deploy the bundle, copy it /opt/keter/incoming on your server.
A recent version of keter had an issue where it would error out with a message along the lines of FileNotExecutable. To work around this I built keter from the most recent GitHub source code. Clone it to your computer then build/install it locally with stack install. Then copy the executable to your server at /opt/keter/bin and create a Systemd service for it:
# /etc/systemd/system/keter.service
[Unit]
Description=Keter
After=network.service
[Service]
Type=simple
ExecStart=/opt/keter/bin/keter /opt/keter/etc/keter-config.yaml
[Install]
WantedBy=multi-user.target