Update Dockerfile with 2.1 sdk (#1622)
Update Dockerfile with 2.1 sdk (#1622)

No longer builds with 2.0:

`The current .NET SDK does not support targeting .NET Core 2.1.  Either target .NET Core 2.0 or lower, or use a version of the .NET SDK that supports .NET Core 2.1.`
Neurosploit/StratisBitcoinFullNode,Aprogiena/StratisBitcoinFullNode,quantumagi/StratisBitcoinFullNode,mikedennis/StratisBitcoinFullNode,fassadlr/StratisBitcoinFullNode,Neurosploit/StratisBitcoinFullNode,mikedennis/StratisBitcoinFullNode,bokobza/StratisBitcoinFullNode,bokobza/StratisBitcoinFullNode,bokobza/StratisBitcoinFullNode,stratisproject/StratisBitcoinFullNode,fassadlr/StratisBitcoinFullNode,quantumagi/StratisBitcoinFullNode,Aprogiena/StratisBitcoinFullNode,stratisproject/StratisBitcoinFullNode,mikedennis/StratisBitcoinFullNode,Neurosploit/StratisBitcoinFullNode,bokobza/StratisBitcoinFullNode,Aprogiena/StratisBitcoinFullNode,mikedennis/StratisBitcoinFullNode