document how to update, closes cypress-io#342

CONTRIBUTING.md

@@ -48,6 +48,25 @@ This will create new folder `included/<Cypress version>`
 4. update [circle.yml](circle.yml) file by running `npm run build` and commit the changes.
 4. open a pull request.
 
+## Tagging the latest image
+
+We build individual base images that match Node versions: `10.18.1`, `12.12.0`, `12.18.2`, etc. We also tag some of the images with major version: `base:10`, `base:12`. We also tag one image `base:latest`. In general, you should use the explicit version like `base:12.18.0` because it guarantees that the Docker image will never be suddenly updated.
+
+To tag new image, like `base:12.18.2` need to do the following from a local machine
+
+```text
+# pull the image to tag
+$ docker pull cypress/base:12.18.2
+# tag that image with major version
+$ docker tag cypress/base:12.18.2 cypress/base:12
+# tag that image with "latest"
+$ docker tag cypress/base:12.18.2 cypress/base:latest
+
+# push the new images (which are the same)
+$ docker push cypress/base:12
+$ docker push cypress/base:latest
+```
+
 ## Bonus: smaller images
 
 Pull request [#83](https://github.com/cypress-io/cypress-docker-images/pull/83) shows how to create smaller Docker images. Follow that PR's advice when creating new images.