Refactor Earthfile for improved build process

Simplified the cloning process by moving Git clone commands outside of
the 'init' target, making them directly accessible. Introduced modular
imports for each repository and aligned them with new build targets to
streamline the development workflow. Upgraded build steps by leveraging
a newer 'sonrhq/builder' image to ensure compatibility and efficiency
during the build-test cycle.

Ref: Project streamline [#123]

Earthfile

@@ -1,10 +1,22 @@
 VERSION 0.7
 PROJECT sonrhq/testnet-1
 
-# init - Initializes the base repository and clones dependencies
-init:
-	LOCALLY
-	GIT CLONE [email protected]:sonrhq/chain.git chain
-	GIT CLONE [email protected]:sonrhq/identity.git identity
-	GIT CLONE [email protected]:sonrhq/rails.git rails
-	GIT CLONE [email protected]:sonrhq/service.git service
+LOCALLY
+GIT CLONE [email protected]:sonrhq/chain.git chain
+GIT CLONE [email protected]:sonrhq/identity.git identity
+GIT CLONE [email protected]:sonrhq/rails.git rails
+GIT CLONE [email protected]:sonrhq/service.git service
+
+
+IMPORT ./chain AS chain
+IMPORT ./identity AS identity
+IMPORT ./rails AS rails
+IMPORT ./service AS service
+
+# build - Initializes the base repository and clones dependencies
+build:
+	FROM sonrhq/builder:latest
+	BUILD identity+test
+	BUILD service+test
+	BUILD chain+build
+	BUILD rails+build