fix: ensure copied source has correct ownership

[jlsalmon]

When using -layout and --opt source_mode=copy followed by docker build from inside a docker container (docker-in-docker) where the executing user is root, the copied source ends up being owned by root and the build fails during the Setup.sh phase due to permissions issues. This patch fixes the issue by ensuring the copied source is owned by the ue4 user.

Specifically, the error is the following:

chmod: changing permissions of 'bin/mcs': Operation not permitted
chmod: changing permissions of 'bin/xbuild': Operation not permitted
Fixing inconsistent case in filenames.
Setting up Mono
Checking dependencies...
Failed to write file '/home/ue4/UnrealEngine/.ue4dependencies.tmp': Access to the path "/home/ue4/UnrealEngine/.ue4dependencies.tmp" is denied.
Result: 1

[TBBle]

Nice catch, thank you.