chore: add Dockerfile and basic CI test

Dockerfile

@@ -0,0 +1,13 @@
+FROM golang:1.18-alpine as builder
+
+RUN apk add --no-cache git make
+WORKDIR /app
+COPY . .
+RUN env CGO_ENABLED=0 go build -a -o dotkatapult -ldflags "-s -w"
+
+FROM scratch
+ENV PORT 8080
+EXPOSE 8080
+WORKDIR /
+COPY --from=builder /app/dotkatapult /
+CMD ["/dotkatapult"]