ci: Add build cache

Cache cargo build directory (`target/`) and npm's `node_modules/`.

.gitlab-ci.yml

@@ -4,6 +4,11 @@ stages:
   - check
   - build
 
+cache: &global_cache
+  paths:
+    - target/
+    - www/node_modules/
+
 check:
   stage: check
   script:
@@ -15,6 +20,10 @@ check:
 
 build-dev:
   stage: build
+  cache:
+    # inherit all global cache settings
+    <<: *global_cache
+    key: merge-requests
   script:
     - ls -al /usr/local/cargo/bin
     - wasm-pack build --dev
@@ -23,6 +32,10 @@ build-dev:
 
 build-release:
   stage: build
+  cache:
+    # inherit all global cache settings
+    <<: *global_cache
+    key: release-build
   script:
     - wasm-pack build
     - cd www