Fixes Issue #14 in upstream. Takes server id as env var

[jonbartels]

I added support and documentation to take SERVER_ID as an environment variable and write it to appdata/server.id.

I tested with:

docker build -t connect_local --build-arg ARTIFACT=https://s3.amazonaws.com/downloads.mirthcorp.com/connect/4.1.1.b303/mirthconnect-4.1.1.b303-unix.tar.gz  .
docker run -e SERVER_ID='UwU-pwease-accept-my-peeawr' -p 8443:8443 connect_local  #check server.id file and log in to MC and see the server ID I set        
docker run -p 8443:8443 connect_local #see the normal autogenerated UUID as server ID        
docker run -e SERVER_ID='' -p 8443:8443 connect_local  #ensure blank is handled and server ID is autogenerated

[jonbartels]

@cturczynskyj one of my devops guys here at Teladoc (Platinum customer, BTW) asked about this EXACT issue. This fixes #14 which you had tagged as triaged. Can I, or my devops goons, provide any specific help to get this PR accepted?

[pladesma]

I've merged the PR to a branch so we can do some internal testing. We should have the code merged to master before the upcoming MC 4.3.0 release.