Fix COPY fails when multiple files are copied to path specified in ENV

[Greendor1234]

Fixes #2946

Description

The problem is that the value inside the variable sd.DestPath is not updated with the value returned from the function ResolveEnvironmentReplacementList, so when the function IsSrcsValid is called to validate the destination there is still the environment variable unsolved.

dest := srcsAndDest.DestPath
...
if !IsDestDir(dest) && totalFiles > 1 {
  return errors.New("when specifying multiple sources in a COPY command, destination must be a directory and end in '/'")
}

So I added a line that updates the sd.DestPath with the value returned from the function ResolveEnvironmentReplacementList

dests, err := ResolveEnvironmentReplacementList([]string{sd.DestPath}, envs, true)
dest := dests[0]
sd.DestPath = dest

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Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

[aaron-prindle]

LGTM, thanks for the fix here @Greendor1234!