Makefile for Docker container is broken

[alb]

Describe the bug
When following the instructions in contrib/docker/ to build one of the comdb2 images you get an error that the make target doesn't exist.

To Reproduce

1. Clone the repository
2. Optional: Run cmake .
3. Run make docker-standalone

Expected behavior
A make target should exist for docker-standalone which build the comdb2-standalone:<version> Docker image

Environment:

• Operating System and Version: Ubuntu 20.04