allow specifying edition of MSSQL to install

[jnahmias]

It would be great if we could specify, as input to the action, the edition of MS Sql Server that we'd like installed.
I believe the MSSQL_PID env variable or /PID setup parameter can be used to configure this during initial setup / docker deployment.
In particular, for the Github Actions use case, I believe "Developer" is an appropriate default (so as not to break existing workflows).

Ref: https://docs.microsoft.com/en-us/sql/linux/sql-server-linux-configure-environment-variables?view=sql-server-2017
Ref: https://docs.microsoft.com/en-us/sql/database-engine/install-windows/install-sql-server-from-the-command-prompt?view=sql-server-2017

[potatoqualitee]

i love this idea! I'll add support when i get a chance