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>

dummy.rs

@@ -0,0 +1 @@
+fn main() {}