Cp: target is not a directory

This should work, add / at the end of cp command to make it clear for it that its dir:

mkdir -p /app/.apt
cp -R $BUILD_DIR/.apt/* /app/.apt/

There is no directory with the name 4.8...Thats your problem :)

Run this before your cp command:

mkdir -p /app/.apt/usr/lib/gcc/x86_64-linux-gnu/$GCC_VERSION

It creates all directories in the given path!