Add dockerfile by popular request (#41, #58)

Rust 1.53 is the oldest compiler that builds shellharden,
and curiously still unbeaten in producing the smallest
executable.

docker/Dockerfile

@@ -0,0 +1,13 @@
+# docker build . -f docker/Dockerfile --tag shellharden:latest
+
+
+FROM rust:1.53.0-alpine AS build
+
+WORKDIR /src
+COPY . .
+RUN cargo build --release
+
+
+FROM scratch
+COPY --from=build /src/target/release/shellharden /shellharden
+ENTRYPOINT ["/shellharden"]