Cache go modules in CI

.gitlab-ci.yml

@@ -2,8 +2,18 @@ stages:
   - test
   - quality-assurance
 
+.go-cache:
+  variables:
+    GOPATH: $CI_PROJECT_DIR/.go
+  before_script:
+    - mkdir -p .go
+  cache:
+    paths:
+      - .go/pkg/mod/
+
 unit_tests:
   image: golang:1.15-alpine
+  extends: .go-cache
   stage: test
   before_script:
     - apk add --no-cache -X http://dl-cdn.alpinelinux.org/alpine/edge/community build-base ffmpeg gcc libsrt-dev
@@ -18,6 +28,7 @@ unit_tests:
 
 linters:
   image: golang:1.15-alpine
+  extends: .go-cache
   stage: quality-assurance
   script:
     - go get -u golang.org/x/lint/golint