CLI service management should allow more flexibility

[andrebriggs]

Creating an epic to capture asks

• [x] Allow flexibility of the location of the Dockerfile #1007
• [x] spk service create should allow for arguments to be passed into the az acr build#1006
• [x] spk should handle pre-existing git ignore files #1081
• [ ] SPK files should not litter root directory #1008
• [x] Custom Dockerfile support #1207
• [x] Allow a project to use an existing variable group #1046