Quick post, mostly for my own future reference, as I couldn’t quite remember how to resize an ext3 filesystem contained in a file.

dd if=/dev/zero of=disk.img bs=1M count=1024 oflag=append conv=notrunc
e2fsck -f disk.img
resize2fs disk.img

The above will append 1GB to the end of the file, then resize the ext2/3 filesystem to take up that newly-added space.