Add new Kaniko Dockerfile for PDNS builder.

Put it besides `Dockerfile-kaniko` until it is clear that one can be
removed.

Dockerfile-kaniko-pdns-builder

@@ -0,0 +1,40 @@
+# From:
+# - https://github.com/GoogleContainerTools/kaniko/blob/main/deploy/Dockerfile
+# - https://hub.docker.com/r/pernodricard/kaniko-alpine/dockerfile
+# - https://stackoverflow.com/a/69251129
+#
+# - `docker build -f Dockerfile-kaniko-pdns-builder -t powerdns/kaniko-pdns-builder .`
+# - `docker push powerdns/kaniko-pdns-builder:latest`
+
+# - `curl --silent https://gcr.io/v2/kaniko-project/executor/tags/list | jq .`
+
+# Kaniko v1.8.0+ has issues with mirror registries than contain a path (like
+# OX's does).
+#FROM gcr.io/kaniko-project/executor:debug AS kaniko
+#FROM gcr.io/kaniko-project/executor:v1.7.0-debug AS kaniko
+#FROM gcr.io/kaniko-project/executor:v1.8.1-debug AS kaniko
+FROM gcr.io/kaniko-project/executor:v1.12.0-debug AS kaniko
+
+FROM alpine:3.11
+
+RUN apk --no-cache add bash binutils coreutils git grep openssh-client perl rsync sed tree
+
+COPY --from=kaniko /kaniko /kaniko
+COPY --from=kaniko /etc/nsswitch.conf /etc/nsswitch.conf
+COPY --from=busybox:musl /bin /busybox
+
+# XXX needed? See: https://github.com/GoogleContainerTools/kaniko/blob/main/deploy/Dockerfile
+# Declare /busybox as a volume to get it automatically in the path to ignore
+#VOLUME /busybox
+
+ENV HOME /root
+ENV USER root
+ENV PATH $PATH:/kaniko:/busybox
+ENV SSL_CERT_DIR /kaniko/ssl/certs
+ENV DOCKER_CONFIG /kaniko/.docker/
+ENV DOCKER_CREDENTIAL_GCR_CONFIG /kaniko/.config/gcloud/docker_credential_gcr_config.json
+ENV GOOGLE_APPLICATION_CREDENTIALS='/kaniko/.docker/config.json'
+
+#WORKDIR /workspace
+
+ENTRYPOINT ["/kaniko/executor"]