Update README.md with DOCKER_HOST env var instructions

I had an error pulling local images. Setting the DOCKER_HOST env var fixed this problem for me. This has has been mentioned in issues such as wagoodman#542 (comment)

Author: benjamin-brady

README.md

@@ -200,6 +200,10 @@ docker run --rm -it \
     -e DOCKER_API_VERSION=1.37 \
     wagoodman/dive:latest <dive arguments...>
 ```
+if you are using an alternative runtime (Colima etc) then you may need to specify the docker host as an environment variable in order to pull local images:
+```bash
+   export DOCKER_HOST=$(docker context inspect -f '{{ .Endpoints.docker.Host }}')
+```
 
 ## CI Integration