Update Dockerfile to build with go1.21 (#150)

Author: gammazero

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.20-bullseye as build
+FROM golang:1.21-bullseye as build
 
 WORKDIR /go/src/motion
 
@@ -12,4 +12,4 @@ RUN CGO_ENABLED=0 go build -o /go/bin/motion ./cmd/motion
 FROM gcr.io/distroless/static-debian12
 COPY --from=build /go/bin/motion /usr/bin/
 
-ENTRYPOINT ["/usr/bin/motion"]
+ENTRYPOINT ["/usr/bin/motion"]