fix(devctr): specify VFS as the storage engine inside devctr

With Docker Engine v28 and before, overlay2 was the default storage
engine used. When launching nested dockerd, an error would be thrown
and dockerd would silently switch to use VFS as the storage engine.

With Docker Engine v29 [1], the default storage engine is now overlayfs,
which does not throw an error on boot, but causes downstream issues when
trying to pull container images. This commit enforces the previous
behaviour, ensuring that new nested Docker versions (which we do not
pin) do not cause errors.

[1]: https://docs.docker.com/engine/release-notes/29/#2900

Signed-off-by: James Curtis <jxcurtis@amazon.co.uk>

Author: JamesC1305

tools/functions

@@ -132,7 +132,7 @@ function validate_version {
 
 # Wait for dockerd to come up in devctr
 prepare_docker() {
-    nohup /usr/bin/dockerd --host=unix:///var/run/docker.sock --host=tcp://127.0.0.1:2375 &
+    nohup /usr/bin/dockerd --host=unix:///var/run/docker.sock --host=tcp://127.0.0.1:2375 --storage-engine=vfs &
 
     # Wait for Docker socket to be created
     timeout 15 sh -c "until docker info; do echo .; sleep 1; done"