Additional fix to installer CSR signername and webhook

It seems that including the signerName in RBAC is optional in 1.17
(and before?), but required as of 1.18.

Including signerName in the CSR itself, is optional in 1.18 but
NOT supported in 1.17 or earlier.

Hence, removing signerName from the CSR creation again. This may need
to be revisited later.

integration/manifests/webhook/webhook-create-signed-cert.sh

@@ -86,7 +86,6 @@ spec:
   groups:
   - system:authenticated
   request: $(cat ${tmpdir}/server.csr | base64 | tr -d '\n')
-  signerName: kubernetes.io/legacy-unknown
   usages:
   - digital signature
   - key encipherment

scm/build/Dockerfile

@@ -32,7 +32,7 @@ RUN scm/build/build.sh \
 # (Intermediate) Stage: Alping base image. Includes common user account and environment
 #                       for netwatcher, svcwatcher, webhook
 #
-FROM alpine:latest AS base-alpine
+FROM alpine:3.11 AS base-alpine
 ARG USERNAME
 ARG UID
 ARG GID
@@ -80,7 +80,7 @@ ENTRYPOINT ["/usr/local/bin/webhook"]
 # Note that unlike the other containers, this needs to run as root as
 # it places CNI plugins into the host's filesystem.
 #
-FROM alpine:latest AS danm-cni-plugins
+FROM alpine:3.11 AS danm-cni-plugins
 
 VOLUME ["/host/cni"]