fix: move docker data dir to /mnt to address e2e test low disk space for clean backport

[weng271190436]

Description of your changes

Was hitting no space left on device in the backport PR e2e tests

Command Output: ctr: failed to extract layer (application/vnd.oci.image.layer.v1.tar sha256:97db83035163d79cf9ecf009c859d2cf93b4d11307f28ad7526beddd84cd1da1) to overlayfs as "extract-771242434-d36z sha256:34af12c2cba687b2ab04efbed891a15eb9adfb188acc24bcd46f25dc44f79f3d": write /var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/15/fs/crdinstaller: no space left on device

Inspected the storage usage in the github action runner and realized that we can use /dev/sda1 (which is mounted to /mnt) for docker.

=== Disk usage after loading images into cluster-2 cluster ===
Filesystem      Size  Used Avail Use% Mounted on
/dev/root        72G   71G  1.4G  99% /
tmpfs           7.9G   84K  7.9G   1% /dev/shm
tmpfs           3.2G  2.2M  3.2G   1% /run
tmpfs           5.0M     0  5.0M   0% /run/lock
/dev/sdb16      881M   62M  758M   8% /boot
/dev/sdb15      105M  6.2M   99M   6% /boot/efi
/dev/sda1        74G  4.1G   66G   6% /mnt
tmpfs           1.6G   12K  1.6G   1% /run/user/1001

The same fix is not applicable to upstream kubefleet repo because upstream CI machine has a big /dev/root with no secondary storage device

Filesystem      Size  Used Avail Use% Mounted on
/dev/root       581G   71G  510G  13% /
tmpfs            32G   84K   32G   1% /dev/shm
tmpfs            13G  2.2M   13G   1% /run
tmpfs           5.0M     0  5.0M   0% /run/lock
/dev/sda16      881M   62M  758M   8% /boot
/dev/sda15      105M  6.2M   99M   6% /boot/efi
tmpfs           6.3G   12K  6.3G   1% /run/user/1001
tmpfs           6.3G   12K  6.3G   1% /run/user/0

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