Speed up docker rebuilds

This should keep all the dependency build artifacts in a cached layer
below the layer that builds our app and links the final binary.

Dockerfile

@@ -2,7 +2,15 @@ FROM rust:1.72-slim-bullseye AS builder
 
 WORKDIR /bebot-build
 
-COPY . ./
+# Build and cache dependencies
+COPY Cargo.toml Cargo.lock ./
+RUN mkdir -p src && echo 'fn main() {}' > src/main.rs
+RUN cargo build --release
+
+# Build and link our app
+RUN rm -rf src
+COPY src ./src
+RUN touch src/main.rs
 RUN cargo build --release
 
 FROM debian:bookworm-slim