feat: first cut at building docker images

- Update .goreleaser to build docker images
- Update Dockerfile to work with goreleaser
- Update examples/docker/awscli/Dockerfile to pull kapow docker image

Co-authored-by: César Gallego <gallego.cesar@gmail.com>
Co-authored-by: Roberto Abdelkader Martínez Pérez <robertomartinezp@gmail.com>
Co-authored-by: Héctor Hurtado <hector.hurtado@bbva.com>

Dockerfile

@@ -1,7 +1,2 @@
-FROM golang:1.14 as build
-
-RUN go get github.com/BBVA/kapow
-RUN CGO_ENABLED=0 GOOS=linux go install github.com/BBVA/kapow
-
 FROM scratch
-COPY --from=build /go/bin/kapow /kapow
+COPY kapow /