feat(ping/rust): Add dummy main to allow for caching of built dependencies (#78)

ping/rust/Dockerfile

@@ -4,7 +4,12 @@ WORKDIR /usr/src/testplan
 RUN apt-get update && apt-get install -y cmake protobuf-compiler
 
 ARG PLAN_PATH="./"
-COPY ./plan/${PLAN_PATH} ./plan
+
+RUN mkdir -p ./plan/src
+COPY ./plan/${PLAN_PATH}/src/main.rs ./plan/src/main.rs
+COPY ./plan/${PLAN_PATH}/Cargo.toml ./plan/Cargo.toml
+COPY ./plan/${PLAN_PATH}/Cargo.lock ./plan/Cargo.lock
+
 RUN cd ./plan/ && cargo build  # Initial build acts as a cache.
 
 ARG GIT_TARGET=""
@@ -13,6 +18,9 @@ RUN if [ ! -z "${GIT_TARGET}" ]; then sed -i "s,^git.*,git = \"https://${GIT_TAR
 ARG GIT_REF=""
 RUN if [ "master" = "${GIT_REF}" ]; then sed -i "s/^rev.*/branch= \"master\"/" ./plan/Cargo.toml; elif [ ! -z "${GIT_REF}" ]; then sed -i "s/^rev.*/rev = \"${GIT_REF}\"/" ./plan/Cargo.toml; fi
 
+COPY ./plan/${PLAN_PATH}/src/lib.rs ./plan/src/lib.rs
+COPY ./plan/${PLAN_PATH}/src/bin/ ./plan/src/bin/
+
 # Build the requested binary: Cargo will update lockfile on changed manifest (i.e. if one of the above `sed`s patched it).
 ARG BINARY_NAME
 RUN cd ./plan/ \

ping/rust/src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("This is a dummy main file that is used for creating a cache layer inside the docker container.")
+}