ci: add a dockerfile

Signed-off-by: SphericalKat <amolele@gmail.com>

Dockerfile

@@ -0,0 +1,7 @@
+FROM rustlang/rust:nightly as build
+
+COPY . .
+
+RUN cargo build --release
+
+CMD ["sh", "-c", "ROCKET_PORT=$PORT ROCKET_KEEP_ALIVE=0 ./target/release/katbin"]