[Backport] - Fix for rc.docker for interfaces with higher index than 0

[ich777]

• Fix error in first if condition which always returned false
• Change name from variables so that it a bit easier to read
• Make sure that custom interfaces with a higher index then 0 are properly rebuilt

This directly address this issue from the Forums.

~~Still needs to be tested by the user who reported it, I will remove the draft status when it's been tested.~~