perf(backend): Cache build of dependencies

When building the Docker image, first compile the project with a fake
main file, namely `dummy.rs`. Once dependencies are built, compile the
actual project by switching back to `src/main.rs`.

Signed-off-by: Lucien Cartier-Tilet <lucien@phundrak.com>

Cargo.toml

@@ -15,3 +15,7 @@ serde = { version = "1.0", features = ["derive"] }
 dotenvy = "0.15"
 gql_client = "1.0.7"
 human-panic = "1.1.0"
+
+[[bin]]
+name = "phuncache"
+path = "src/main.rs"