cnb-overlay.txt

$ docker run --rm --privileged -it heroku/heroku:18-build bash
root@f1442b4c03bb:/# mkdir -p /tmp/overlay && mount -t tmpfs tmpfs /tmp/overlay && mkdir -p /tmp/overlay/{usr,work}

root@f1442b4c03bb:/# mount -t overlay -o lowerdir=/usr,upperdir=/tmp/overlay/usr/,workdir=/tmp/overlay/work/ overlay /usr/

root@f1442b4c03bb:/# apt update
...

root@f1442b4c03bb:/# apt install tree
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following NEW packages will be installed:
  tree
0 upgraded, 1 newly installed, 0 to remove and 29 not upgraded.
Need to get 40.7 kB of archives.
After this operation, 105 kB of additional disk space will be used.
Get:1 http://archive.ubuntu.com/ubuntu bionic/universe amd64 tree amd64 1.7.0-5 [40.7 kB]
Fetched 40.7 kB in 0s (115 kB/s)
debconf: unable to initialize frontend: Dialog
debconf: (No usable dialog-like program is installed, so the dialog based frontend cannot be used. at /usr/share/perl5/Debconf/FrontEnd/Dialog.pm line 76, <> line 1.)
debconf: falling back to frontend: Readline
Selecting previously unselected package tree.
(Reading database ... 35993 files and directories currently installed.)
Preparing to unpack .../tree_1.7.0-5_amd64.deb ...
Unpacking tree (1.7.0-5) ...
Setting up tree (1.7.0-5) ...

root@f1442b4c03bb:/# tree /tmp/overlay/usr/
/tmp/overlay/usr/
|-- bin
|   `-- tree
`-- share
    `-- doc
        `-- tree
            |-- changelog.Debian.gz
            `-- copyright

4 directories, 3 files