DevContainer move to single dockerfile from compose

[lawrencegripper]

Hi,

This is a quick set of tweaks to make the devcontainer a bit more friendly and easy to get started with on different platforms (linux, mac etc).

Some of them are quite opinionated so feel free to push back on changes if they don't fit general workflow the team is currently using.

Changes

• Add alias for Kubectl
• Add kubectx to allow easy switch between kind cluster and remote clusters
• Install git prompt and bash completion for kubectl, kubectx etc

[lawrencegripper]

@Azadehkhojandi Thanks for the review, I've fixed those ones up now. Sorry for the delayed response my daughter has been unwell so I've been away.