Fix some serving samples' Dockerfile to run as non-root user

[prushh]

Some fixes related to knative/serving#14566

Proposed Changes

Defined a new user in the Dockerfile of the following serving sample projects to run the containers as a non-root user:

• cloudevents
  • cloudevents-dotnet
  • cloudevents-go
  • cloudevents-nodejs
  • cloudevents-rust
  • cloudevents-spring
  • cloudevents-vertx
• gitwebhook-go
• grpc-ping-go
• hello-world
  • helloworld-csharp
  • helloworld-go
  • helloworld-java-spark
  • helloworld-java-spring
  • helloworld-kotlin
  • helloworld-nodejs
  • helloworld-php
  • helloworld-python
  • helloworld-ruby
  • helloworld-scala
  • helloworld-shell
• knative-routing-go
• kong-routing-go
• multi-container
  • servingcontainer
  • sidecarcontainer
• secrets-go

All items marked in the task list have been built and tested using only Docker (no Knative Service) and they work as expected.

I updated the README.md file of each project when necessary (ex. the Dockerfile if specified, markdown warnings, etc.).

In all Dockerfiles using the gcr.io/distroless repository for the last stage image, I didn't know which set of commands to use (useradd/groupadd vs adduser/addgroup).