Document how to upgrade a bundle using digest (getporter#2352)

Author: VinozzZ

REVIEWING.md

@@ -108,6 +108,7 @@ of Porter. When you are asked to cut a new release, here is the process:
     git tag VERSION -a -m ""
     git push --tags
     ```
+    If the CI build failed to build for the release, fix the problem first. Then increment the PATCH version, e.g. v0.7.0->v0.7.1, and go through the above steps again to publish the binaries. It's often a good pratice to finish the release first before updating any of our docs that references the latest release.
 
 1. Generate some release notes and put them into the release on GitHub.
    - Go to Porter Github repository and find the newly created release tag. You should see a

docs/content/quickstart/_index.md

@@ -116,18 +116,42 @@ History:
 ## Upgrade the Installation
 
 To upgrade the resources managed by the bundle, use `porter upgrade`.
+### Upgrade using a version tag
 Most bundles are written such that a specific version of the bundle corresponds to a specific version of an application.
 So to upgrade the application to a new version you need to specify a newer version of the bundle.
 
 ```console
-$ porter upgrade porter-hello --reference ghcr.io/getporter/examples/porter-hello:v0.2.0
+$ porter upgrade porter-hello --version 0.2.0
 upgrading porter-hello...
 executing upgrade action from porter-hello (installation: porter-hello)
 Upgrade Hello World
 Upgraded to World 2.0
 execution completed successfully!
 ```
 
+### Upgrade using digest
+
+When working in production environment, we highly recommend you to reference the bundle using its digest instead of tag. We used tags in our docs for simplicity, but tags can be overwritten which results in unexpected outcomes from upgrading an bundle.
+For deterministic and repeatable deployments, use digests instead of tags to ensure that you deploy exactly what you intended.
+
+```console
+$ porter upgrade porter-hello --reference ghcr.io/getporter/examples/porter-hello@sha256:276b44be3f478b4c8d1f99c1925386d45a878a853f22436ece5589f32e9df384
+_
+upgrading porter-hello...
+executing upgrade action from porter-hello (installation: porter-hello)
+Upgrade Hello World
+Upgraded to World 2.0
+execution completed successfully!
+```
+
+## Troubleshooting
+
+If you received an `invalid media type` error like below, check that you are referencing the digest for the bundle and not the installer image.
+
+```plain
+unable to pull bundle: invalid media type "application/vnd.docker.distribution.manifest.v2+json" for bundle manifest
+```
+
 ## Cleanup
 
 To clean up the resources installed from the bundle, use the `porter uninstall` command.