[Gitpod CLI] gp rebuild improvements

[andreafalzetti]

Description

Follow up PR of things we overlooked in the initial PR:

1. add event outcome,
2. use the tmp dockerfile
3. build from checkout location context (using . won't work if the user is inside a sub-directory)
4. the Stderr coming from docker run should not be propagated outside of the container because it can be anything (e.g user typing exit 1) which currently gets logged as an error from gp rebuild
5. return an explicit nil error when there is not real error

success event	user error	system error

Related Issue(s)

Fixes #

How to test

1. Start a workspace
2. Create a Dockerfile which used COPY and relies on location, make sure files are copied correctly from the workspace checkout location
3. Try using an image which does not have bash installed (ie. busybox)
4. Try typing exit 1 to leave the container
5. test each branch changed

Release Notes

NONE 

Documentation

Werft options:

• /werft with-local-preview
  If enabled this will build install/preview
• /werft with-preview
• /werft with-large-vm
• /werft with-integration-tests=all
  Valid options are all, workspace, webapp, ide, jetbrains, vscode, ssh
• /werft analytics=segment

[akosyakov]

Code wise it seems to be alright, I have not tried

/hold

@andreafalzetti @felladrin please merge when you are sure

[felladrin]

Reviewed and tested! ✅