Updated readme to specify repository:tag requirement for Docker images

Author: ubaniabalogun

README.md

@@ -126,7 +126,7 @@ The plugin configurations are simple:
 | globalRequirements | A list of paths to files containing service-level pip requirements.                                                                                                                                                | Yes                                                        |
 | globalIncludes     | A list of paths to folders containing service-level code files (i.e. code common to all functions). Only the folders contents will be packaged, not the folder itself. Paths to files are not currently supported. | Yes                                                        |
 | useDocker          | Boolean indicating whether to package pip dependencies using Docker. Set this to true if your project uses platform-specific compiled libraries like numpy. Requires a Docker installation.                        | Yes. Defaults to `false`                                   |
-| dockerImage        | The Docker image to use to compile functions if `useDocker` is set to `true`. If the image doesn't exist on the system, it will be downloaded. The initial download may take some time.                            | Yes. Defaults to `lambci/lambda:build-${provider.runtime}` |
+| dockerImage        | The Docker image to use to compile functions if `useDocker` is set to `true`. Must be specified as `repository:tag`. If the image doesn't exist on the system, it will be downloaded. The initial download may take some time.                            | Yes. Defaults to `lambci/lambda:build-${provider.runtime}` |
 | containerName      | The desired name for the Docker container.                                                                                                                                                                         | Yes. Defaults to `serverless-package-python-functions`     |
 
 At the function level, you: