UPDATE Feb 2021:
Nowadays I would recommend just copying the firmware deb files to the USB stick directly without going through this mess, just as this answer suggests in this thread. I placed my iwlwifi package in both the root of the USB stick, as well as the "firmware" folder. Don't know which one is the right one, but it works now. Thanks!
If you want to create a bootable Debian USB stick with non-free drivers included, you can copy the ISO to the drive directly then create a new partition where you include the .deb files of whatever drivers you need.
First make sure you're not copying to the wrong device by checking the output of dmesg after connecting your flash drive.
Now copy debian on to your USB stick:
cat debian.iso > /dev/sdb; sync
Then create a new partition by using fdisk:
fdisk /dev/sdb
Use these commands to configure the new partition using fdisk (this assumes you already have 1 partition on it):
n (Create new partition)
p (Partition type: Primary)
2 (Partition number)
Enter (select default values for first sector)
Enter (select default values for last sector)
t (Change partition type)
2 (Partition number)
6 (Hex code for type FAT16)
w (to write changes to disk)
Now we inform the OS of partition table changes (apt-get install parted):
partprobe
Format the partition with vfat:
mkfs.vfat /dev/sdb2
Mount the new partition:
mount /dev/sdb2 /mnt
Now you can copy whatever .deb files you want the installer to detect during installation, for example:
cp firmware-bnx2_0.28+squeeze1_all.deb /mnt
Make sure all data is written to disk (probably unnecessary):
sync
Unmount USB drive:
umount /mnt
Done!
Now the Debian installer will detect all the drivers you added to the new partition and use them. In my case I needed non-free NIC drivers for a Dell PowerEdge server.
Best Answer
Instructions for installing dependencies and compiling from source can be found on their github: https://github.com/anbox/anbox