[20.10] test containerd binary v1.7.0-rc.3

Once we publish packages for containerd.io 1.7.0, existing daemons may also
update to that version, so opening this to test if CI is still happy with
this version.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

hack/dockerfile/install/containerd.installer

@@ -15,7 +15,7 @@ set -e
 # the binary version you may also need to update the vendor version to pick up
 # bug fixes or new APIs, however, usually the Go packages are built from a
 # commit from the master branch.
-: "${CONTAINERD_VERSION:=v1.6.16}"
+: "${CONTAINERD_VERSION:=v1.7.0-rc.3}"
 
 install_containerd() (
 	echo "Install containerd version $CONTAINERD_VERSION"