doc: Use docker.io instead of quay.io

There was some confusion among new contributors whether they should use
the Cilium image from docker.io or quay.io for running integration tests
locally. Technically both are fine, but changing the documentation to
use the Cilium image from docker.io to signify that it is considered a
bit more official.

Signed-off-by: Michi Mutsuzaki <michi@isovalent.com>

Documentation/contributing/testing/e2e.rst

@@ -460,7 +460,7 @@ An example invocation is
 
 ::
 
-  CNI_INTEGRATION=eks K8S_VERSION=1.13 ginkgo --focus="K8s*" -noColor -- -cilium.provision=false -cilium.kubeconfig=`echo ~/.kube/config` -cilium.image="quay.io/cilium/cilium:latest" -cilium.operator-image="quay.io/cilium/operator:latest" -cilium.passCLIEnvironment=true
+  CNI_INTEGRATION=eks K8S_VERSION=1.13 ginkgo --focus="K8s*" -noColor -- -cilium.provision=false -cilium.kubeconfig=`echo ~/.kube/config` -cilium.image="docker.io/cilium/cilium:latest" -cilium.operator-image="docker.io/cilium/operator:latest" -cilium.passCLIEnvironment=true
 
 GKE (experimental)
 ^^^^^^^^^^^^^^^^^^^^^^
@@ -482,7 +482,7 @@ cluster.
 
 ::
 
-  CNI_INTEGRATION=gke K8S_VERSION=1.13 ginkgo -v --focus="K8sDemo*" -noColor -- -cilium.provision=false -cilium.kubeconfig=`echo ~/.kube/config` -cilium.image="quay.io/cilium/cilium:latest" -cilium.operator-image="quay.io/cilium/operator:latest" -cilium.passCLIEnvironment=true
+  CNI_INTEGRATION=gke K8S_VERSION=1.13 ginkgo -v --focus="K8sDemo*" -noColor -- -cilium.provision=false -cilium.kubeconfig=`echo ~/.kube/config` -cilium.image="docker.io/cilium/cilium:latest" -cilium.operator-image="docker.io/cilium/operator:latest" -cilium.passCLIEnvironment=true
 
 .. note:: The kubernetes version defaults to 1.13 but can be configured with
           versions between 1.13 and 1.15. Check with ``kubectl version`` 
@@ -500,7 +500,7 @@ cluster.
 
 ::
 
-  CNI_INTEGRATION=eks K8S_VERSION=1.14 ginkgo -v --focus="K8sDemo*" -noColor -- -cilium.provision=false -cilium.kubeconfig=`echo ~/.kube/config` -cilium.image="quay.io/cilium/cilium:latest" -cilium.operator-image="quay.io/cilium/operator:latest" -cilium.passCLIEnvironment=true
+  CNI_INTEGRATION=eks K8S_VERSION=1.14 ginkgo -v --focus="K8sDemo*" -noColor -- -cilium.provision=false -cilium.kubeconfig=`echo ~/.kube/config` -cilium.image="docker.io/cilium/cilium:latest" -cilium.operator-image="docker.io/cilium/operator:latest" -cilium.passCLIEnvironment=true
 
 Be sure to pass ``--cilium.passCLIEnvironment=true`` to allow kubectl to invoke ``aws-iam-authenticator``