rewrite dockerfile / deployment

Dockerfile

@@ -1,10 +1,21 @@
-FROM rust:1.43 as builder
+FROM rust:1.47 as builder
 
 WORKDIR /usr/src/sarcastifybot
+
+# Avoid having to install/build all dependencies by copying
+# the Cargo files and making a dummy src/main.rs
+COPY Cargo.toml .
+COPY Cargo.lock .
+RUN mkdir -p src && echo "fn main() {}" > src/main.rs
+RUN cargo build --release
+
+# We need to touch our real main.rs file or else docker will use
+# the cached one.
 COPY . .
-RUN cargo install --path .
+RUN touch src/main.rs
+RUN cargo build --release
 
 FROM debian:buster
 RUN apt-get update && apt-get install -y openssl ca-certificates
-COPY --from=builder /usr/local/cargo/bin/sarcastifybot /usr/local/bin/sarcastifybot
+COPY --from=builder /usr/src/sarcastifybot/target/release/sarcastifybot /usr/local/bin/sarcastifybot
 CMD ["sarcastifybot"]