k8s: Update Alpine to 3.18.

images/k8s/Dockerfile

-FROM docker.io/library/alpine:3.15
+FROM docker.io/library/alpine:3.18
 # hadolint ignore=DL3018
 RUN wget -q https://apk.cloudposse.com/ops@cloudposse.com.rsa.pub -O /etc/apk/keys/ops@cloudposse.com.rsa.pub && \
     echo "https://apk.cloudposse.com/3.15/vendor" >> /etc/apk/repositories && \