Utilize Docker cache to speed up building

Signed-off-by: Vasilis Tsiligiannis <acinonyx@openwrt.gr>

.gitlab-ci.yml

@@ -56,7 +56,9 @@ docker:
     - apk --update add py-pip
     - pip install docker-compose
   script:
-    - docker-compose build --no-cache --pull
+    - CACHE_IMAGE="${CI_REGISTRY_IMAGE:+$CI_REGISTRY_IMAGE/satnogs-db:$CI_COMMIT_REF_NAME}"; export CACHE_IMAGE
+    - docker-compose -f docker-compose.yml -f docker-compose.cache.yml pull cache_image || true
+    - docker-compose -f docker-compose.yml -f docker-compose.cache.yml build --pull
     - |
       [ -z "$CI_REGISTRY_IMAGE" ] || {
           docker login -u $CI_REGISTRY_USER -p $CI_JOB_TOKEN $CI_REGISTRY

docker-compose.cache.yml

@@ -0,0 +1,12 @@
+version: '3.2'
+services:
+  cache_image:
+    image: '$CACHE_IMAGE'
+  web:
+    build:
+      cache_from:
+        - '$CACHE_IMAGE'
+  celery:
+    build:
+      cache_from:
+        - '$CACHE_IMAGE'