build_docker.sh: use github.com/tailscale/mkctr instead of docker

Signed-off-by: Maisem Ali <maisem@tailscale.com>

Dockerfile

@@ -56,6 +56,5 @@ RUN GOARCH=$TARGETARCH go install -tags=xversion -ldflags="\
       -X tailscale.com/version.GitCommit=$VERSION_GIT_HASH" \
       -v ./cmd/tailscale ./cmd/tailscaled
 
-FROM alpine:3.14
-RUN apk add --no-cache ca-certificates iptables iproute2 ip6tables
+FROM ghcr.io/tailscale/alpine-base:3.14
 COPY --from=build-env /go/bin/* /usr/local/bin/