Security features for `DockerExec`

• made the test on /certs directory a warning (not error), as we might not want to use DockerExec only with one proxy stack
• added tests for having docker and docker-compose executables installed on the host machine
• removed which usage and tested executables with command instead

Fixed the self-update command

• fixed the DockerExec self-update command
• added a LICENSE file
• mentioned the nginx-proxy project in readme, which our stack is based on

First release for the Docker-Proxy-Stack

For the current state, simply read the readme.md.