Update alpine:latest Docker digest to 21dc606 (#104)

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

Dockerfile

@@ -15,7 +15,7 @@ COPY *.go ./
 RUN go build -o confluence-gardner
 
 # final stage
-FROM alpine:latest@sha256:1e42bbe2508154c9126d48c2b8a75420c3544343bf86fd041fb7527e017a4b4a
+FROM alpine:latest@sha256:21dc6063fd678b478f57c0e13f47560d0ea4eeba26dfc947b2a4f81f686b9f45
 COPY --from=build-env /go/src/confluence-gardner/confluence-gardner  /usr/local/bin/confluence-gardner
 RUN mkdir -p /output
 ENV DIRECTORY /output