docker: fix stale build and copy build.rs in dep-warming layer

Author: Algorithm5838

Dockerfile.alpine

@@ -11,14 +11,14 @@ RUN apk add --no-cache musl-dev
 WORKDIR /redlib
 
 # download (most) dependencies in their own layer
-COPY Cargo.lock Cargo.toml ./
+COPY Cargo.lock Cargo.toml build.rs ./
 RUN mkdir src && echo "fn main() { panic!(\"why am i running?\") }" > src/main.rs
 RUN cargo build --release --locked --bin redlib
 RUN rm ./src/main.rs && rmdir ./src
 
 # copy the source and build the redlib binary
 COPY . ./
-RUN cargo build --release --locked --bin redlib
+RUN touch src/main.rs && cargo build --release --locked --bin redlib
 RUN echo "finished building redlib!"
 
 ########################

Dockerfile.ubuntu

@@ -10,14 +10,14 @@ FROM rust:${RUST_BUILDER_VERSION} AS builder
 WORKDIR /redlib
 
 # download (most) dependencies in their own layer
-COPY Cargo.lock Cargo.toml ./
+COPY Cargo.lock Cargo.toml build.rs ./
 RUN mkdir src && echo "fn main() { panic!(\"why am i running?\") }" > src/main.rs
 RUN cargo build --release --locked --bin redlib
 RUN rm ./src/main.rs && rmdir ./src
 
 # copy the source and build the redlib binary
 COPY . ./
-RUN cargo build --release --locked --bin redlib
+RUN touch src/main.rs && cargo build --release --locked --bin redlib
 RUN echo "finished building redlib!"
 
 ########################