Update to Edition 2021 and fix docker build

Dockerfile

@@ -1,10 +1,11 @@
-FROM rust:1.55 as cargo-build
+FROM rust:1.56 as cargo-build
 
 WORKDIR /usr/src/onetun
 COPY Cargo.toml Cargo.toml
 
 # Placeholder to download dependencies and cache them using layering
 RUN mkdir src/
+RUN touch src/lib.rs
 RUN echo "fn main() {println!(\"if you see this, the build broke\")}" > src/main.rs
 RUN cargo build --release
 RUN rm -f target/x86_64-unknown-linux-musl/release/deps/myapp*