Cache intermediate layers of container build

Author: tpoliaw

.github/workflows/_container.yml

@@ -29,6 +29,8 @@ jobs:
           # Need load and tags so we can test it below
           load: true
           tags: test_tag
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
 
       - name: Test cli works in cached runtime image
         run: docker run --rm test_tag --version

Dockerfile

@@ -6,8 +6,15 @@ RUN rustup target add x86_64-unknown-linux-musl && \
     apt-get install -y musl-tools musl-dev && \
     update-ca-certificates
 
+# Build an empty project with only the Cargo files to improve the cache
+# performance of the container build. The src directory is expected to
+# change most frequently invalidating later caches so building early
+# allows built dependencies to be cached.
+RUN mkdir src && echo 'fn main() {}' > src/main.rs
 COPY ./Cargo.toml ./Cargo.toml
 COPY ./Cargo.lock ./Cargo.lock
+RUN cargo build --release --target x86_64-unknown-linux-musl
+
 COPY ./src ./src
 
 RUN cargo build --release --target x86_64-unknown-linux-musl