Switch to ext4 for generated .img files (#374)

Ubuntu has been using [ext4 as the default since Ubuntu 9.10](https://wiki.ubuntu.com/KarmicKoala/TechnicalOverview#ext4_by_default) (2009), and the ext3 driver in the Linux kernel was [removed](https://lwn.net/Articles/651645/) in [version 4.3](https://kernelnewbies.org/Linux_4.3#The_Ext3_filesystem_has_been_removed) (2015), so internally, the ext4 driver has been mounting the ext3 images as loop devices for a long time already on Heroku runtime instances.

All mounts that overlay the read-only root filesystem in containers, such as `/app`, have also been ext4 ever since it became the default:

    % heroku run -- mount | head
    Running mount on sushi... up, run.3567
    /dev/loop17 on / type ext3 (ro,nosuid,nodev,nodiratime,relatime)
    none on /dev type tmpfs (rw,relatime,size=492k,mode=755,inode64)
    /dev/mapper/evg0-evol0 on /app type ext4 (rw,relatime)
    /dev/mapper/evg0-evol0 on /tmp type ext4 (rw,relatime)
    /dev/mapper/evg0-evol0 on /var/tmp type ext4 (rw,relatime)
    none on /proc type proc (rw,relatime)
    none on /dev type tmpfs (rw,nosuid,noatime,nodiratime,size=1024k,mode=755,inode64)
    none on /dev/shm type tmpfs (rw,nosuid,nodev,relatime,size=65536k,nr_inodes=2048,mode=700,uid=5790,gid=5790,inode64)
    /dev/mapper/evg0-evol0 on /etc/passwd type ext4 (ro,relatime)
    /dev/mapper/evg0-evol0 on /etc/group type ext4 (ro,relatime)

GUS-W-21298571

Author: dzuelke

tools/bin/capture-docker-stack

@@ -40,7 +40,7 @@ display "Modifying image directories and files"
 install-heroku-files "${IMG_MNT}" |& indent
 
 display "Unmounting image"
-df --human-readable "${IMG_MNT}" |& indent
+df --sync --human-readable "${IMG_MNT}" |& indent
 umount "${IMG_MNT}" |& indent
 
 display "SHA256ing and gzipping image"

tools/bin/make-filesystem-image

@@ -9,7 +9,7 @@ DOCKER_IMAGE_SIZE_IN_MB="$2"
 # of the original Docker image to avoid either wasting space or having the later tar extraction
 # step fail with out of disk space errors. The image will be mounted read-only at runtime, so
 # does not need free space for app files (separate mounts are used for those). The multiplier
-# here is to account for the 5-6% loss of usable space due to ext3 filesystem overhead, as well
+# here is to account for the 5-6% loss of usable space due to ext4 filesystem overhead, as well
 # as to ensure a few MB additional free space headroom.
 IMG_SIZE_IN_MB=$((DOCKER_IMAGE_SIZE_IN_MB * 107 / 100))
 
@@ -23,13 +23,11 @@ mkdir -p "$(dirname "$IMG")"
 # https://unix.stackexchange.com/a/121888
 fallocate --length "${IMG_SIZE_IN_MB}MiB" "${IMG}"
 
-# Format that file as an ext3 filesystem.
+# Format that file as an ext4 filesystem.
 # The `-T` argument forces the 'default' config profile to be used, since otherwise if the filesystem size
 # is less than 512 MB (as is the case for Heroku-24's run image) the 'small' profile would be used instead.
 # The `-m` argument reduces reserved-blocks-percentage from its default of 5% to 1%.
-# TODO: Switch to calling `mkfs.ext3` or `mke2fs -t ext3` since the `mkfs` alias is deprecated:
-# https://manpages.ubuntu.com/manpages/jammy/en/man8/mkfs.8.html
-mkfs -t ext3 -T default -m 1 -v "$IMG"
+mkfs.ext4 -T default -m 1 -v "$IMG"
 
 # Adjust the filesystem parameters for improved performance on runtime instances.
 # The `-c` and `-i` arguments disable automatic filesystem checks, which are otherwise run based