Merge pull request #653 from w0rmr1d3r/document_issue_638

Updating documentation on how to build the docker image (issue #638)

README.md

@@ -48,6 +48,12 @@ brew install gitleaks
 
 ##### Docker
 
+Building the image after cloning the repo:
+```bash
+make dockerbuild
+```
+
+Using the image from DockerHub:
 ```bash
 # To just pull the image
 docker pull zricethezav/gitleaks:latest