This is a fully functional SFTP/SSH server for the cost of only 17.7MB!
The default configuration has a non-privileged user sftp with password DockerPassSFTP and is exposed on port 2222.
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The following command will use physical port 2222, detach the container to run in the background, and name the container sftp:
$ docker run -p 2222:2222 -d --name sftp tailsdotcom/docker-sftp
You can then run sftp -P 2222 sftp@localhost to connect to the container.