Update alpine Docker tag to v3.21.0 (#970)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Dockerfile

@@ -9,7 +9,7 @@ COPY e2e_test.go .
 COPY pkg ./pkg
 RUN go test -race -v ./...
 RUN GOARCH="$(echo "${TARGETPLATFORM}" | cut -d/ -f2)" CGO_ENABLED=0 GOOS=linux go build -o /usr/local/bin/aws-ses-pop3-server
-FROM alpine:3.20.3@sha256:1e42bbe2508154c9126d48c2b8a75420c3544343bf86fd041fb7527e017a4b4a as runner
+FROM alpine:3.21.0@sha256:e323a465c03a31ad04374fc7239144d0fd4e2b92da6e3e0655580476d3a84621 as runner
 COPY --from=builder /usr/local/bin/aws-ses-pop3-server /usr/local/bin/aws-ses-pop3-server
 RUN addgroup --gid 1767 appgroup && \
     adduser --disabled-password --gecos '' --no-create-home -G appgroup --uid 1767 appuser