Dockerfile - improved caching!

To learn more: https://docs.docker.com/build/cache/ .

docker/Dockerfile

@@ -4,18 +4,23 @@ FROM nixos/nix
 # Set the working directory in the container
 WORKDIR /usr/src/app
 
-# Copy the current directory contents into the container at /usr/src/app
-COPY . .
+# For better cashing, first copy only files that maybe required
+# for building nix environment.
+COPY *.nix manifests patch script .
 
 # Install the Fuel tools using Nix
 RUN nix-channel --add https://nixos.org/channels/nixos-unstable nixos \
     && nix-channel --update \
     && nix --extra-experimental-features "nix-command flakes" build github:FuelLabs/fuel.nix#fuel
 # Correct URL to repository is : https://github.com/FuelLabs/fuel.nix/
 
+# Now, once we finished long, expensive nix building, we can
+# copy over all contents of the current directory into the container at /usr/src/app 
+COPY . .
+
 # Define environment variable for Nix
 ENV ENV="/root/.nix-profile/etc/profile.d/nix.sh"
 
 # Run a shell when the container launches
-# CMD ["/bin/sh"]
 CMD ["/bin/bash"]
+