make the agent image 4x smaller thanks to multi stage build (backport #90)

[mergify]

This is an automatic backport of pull request #90 done by Mergify.
Cherry-pick of 4d0209d has failed:

On branch mergify/bp/galactic/pr-90
Your branch is up to date with 'origin/galactic'.

You are currently cherry-picking commit 4d0209d.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   micro-ROS-Agent/micro-ros_entrypoint.sh

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   base/Dockerfile
	both modified:   micro-ROS-Agent/Dockerfile

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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