sshd_config - make 'Port' config key overridable using an env var

[FXHibon]

As atmoz/sftp is a docker image by design, one can already simply change the exposed port of the running server using the appropriate docker run options, to map the internal port 22 to any external port.
It works very well in the majority of cases, as I've been able to experiment it myself.

However, there are a few cases where it is either not wanted or not possible to use this port mapping features.

For example, if you use atmoz/sftp in a CircleCI build, you won't be able to change the port mapping ( https://discuss.circleci.com/t/change-db-containers-ports/30234/6 ), even while the port 22 is reserved by CircleCI, and not usable to containers launched in the tests.

I've read about the custom scripts located in /etc/sftp.d ( https://github.com/atmoz/sftp#execute-custom-scripts-or-applications ) that might already allow one to override everything we want, but that might sounds a bit overengineered to provide a custom executable bash scripts to update a simple config key (this is opinionated, I'm ready to hear any different opinions on this)

Given the reasons listed above, I feel that it is reasonable to allow a user to override the SSHD port using a simple environment variable.

This Pull Request suggests an implementation of this.

[adrian-amaglio]

Hello,
I made it work with the current image by adding this script in /etc/sftp.d/
It allow you to change any SSHD configuration parameter via env vars.
For instance, you can set the ssh port by setting the SSHD_CONFIG_Port env var.

#!/bin/bash    
    
config_file=/etc/ssh/sshd_config    
    
# Browse env var        
while IFS='=' read key value ; do    
    # Until we get this specific prefix        
    if [[ "$key" == SSHD_CONFIG_* ]] ; then    
        key=${key#SSHD_CONFIG_}                    
                                       
        # If the config instruction exists in the config file    
        if [ -n "$(grep -F "$key" "$config_file")" ] ; then      
            # Replace it                                         
            sed -i "s#^$key .*\$#$key $value#" "$config_file"      
        else        
            # Else, add it                                           
            echo "$key $value" >> "$config_file"    
        fi        
    fi            
done < <(env)