Add Dockerfile

The dockerfile is used both as the dev container as well as the
production container. The production container is built with only the
executable binary and dependencies.

Dockerfile

@@ -0,0 +1,20 @@
+FROM docker.io/library/golang:1.19-bullseye as dev
+
+FROM dev as intermediate
+
+COPY . /build
+WORKDIR /build
+RUN go build -o doddns && \
+    ldd doddns | tr -s '[:blank:]' '\n' | grep '^/' | \
+    awk '{printf("%s\n", $1); system("readlink -f " $1)}' | \
+    xargs -I % sh -c 'mkdir -p $(dirname deps%); cp % deps%;'
+
+FROM scratch as prod
+
+COPY --from=intermediate /build/deps /
+COPY --from=intermediate /build/doddns /doddns
+COPY --from=intermediate /etc/ssl/certs/* /etc/ssl/certs/
+COPY --from=intermediate /usr/share/ca-certificates /usr/share/ca-certificates
+
+WORKDIR /
+ENTRYPOINT ["/doddns"]