Update alpine Docker tag to v3.21.1 (#975)

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:3.21.0@sha256:21dc6063fd678b478f57c0e13f47560d0ea4eeba26dfc947b2a4f81f686b9f45 as runner
+FROM alpine:3.21.1@sha256:b97e2a89d0b9e4011bb88c02ddf01c544b8c781acf1f4d559e7c8f12f1047ac3 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