Attempt to remove unknown arch from containerized builds (#56)

Dockerfile

@@ -1,9 +1,9 @@
-FROM golang:alpine AS build
+FROM --platform=$BUILDPLATFORM golang:alpine AS build
 COPY . .
 ARG VERSION
 RUN go build -trimpath -o /app/ \
     -ldflags "-s -w -X github.com/codereaper/lane/cmd.versionString=$VERSION"
 
-FROM scratch
+FROM --platform=$BUILDPLATFORM scratch
 COPY --from=build /app/lane /
 ENTRYPOINT [ "/lane" ]