Makefile for Docker container is broken

**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:` Docker image

**Environment:**
- Operating System and Version: Ubuntu 20.04