Bump docker.io/alpine image from 3.20.3 to v3.21.0 (#45)

Co-authored-by: dghubble-renovate[bot] <119624128+dghubble-renovate[bot]@users.noreply.github.com>

Dockerfile

@@ -1,10 +1,10 @@
-FROM docker.io/alpine:3.20.3 AS builder
+FROM docker.io/alpine:3.21.0 AS builder
 ARG ARCH
 ARG CNI_VERSION
 COPY scripts /scripts
 RUN /scripts/fetch-cni.sh
 
-FROM docker.io/alpine:3.20.3
+FROM docker.io/alpine:3.21.0
 LABEL maintainer="Dalton Hubble <[email protected]>"
 LABEL org.opencontainers.image.title="flannel-cni",
 LABEL org.opencontainers.image.source="https://github.com/poseidon/flannel-cni"