chore(dockerfile): reduce packaging time (backport #3230)

[mergify]

Which issue(s) this PR fixes:

Issue longhorn/longhorn#8744

What this PR does / why we need it:

The packages aren't updated frequently, so the content of the image layer usually remains unchanged.
To avoid unnecessary rebuilding of this layer, copy the longhorn-manager binaries after installing the packages.

Special notes for your reviewer:

Additional documentation or context

---

This is an automatic backport of pull request #3230 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 201e11a has failed:

On branch mergify/bp/v1.6.x/pr-3230
Your branch is up to date with 'origin/v1.6.x'.

You are currently cherry-picking commit 201e11a8.
  (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)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   package/Dockerfile

no changes added to commit (use "git add" and/or "git commit -a")

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