Dockerfile: reduce size, add linting

.gitlab-ci.yml

-image:
-  name: gcr.io/kaniko-project/executor:debug
-  entrypoint: [""]
+stages:
+  - lint
+  - build
+
+lint:
+  stage: lint
+  image: hadolint/hadolint:latest-debian
+  script:
+    - hadolint Dockerfile
 
 build:
   stage: build
+  image:
+    name: gcr.io/kaniko-project/executor:debug
+    entrypoint: [""]
   script:
     - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /kaniko/.docker/config.json
     - /kaniko/executor --context $CI_PROJECT_DIR --dockerfile $CI_PROJECT_DIR/Dockerfile --destination $CI_REGISTRY_IMAGE:$CI_COMMIT_SHORT_SHA --destination $CI_REGISTRY_IMAGE:latest

Dockerfile

@@ -7,8 +7,9 @@ ENV ACCEPT_KEYWORDS="~amd64" \
     EMERGE_DEFAULT_OPTS="--jobs=4 --quiet" \
     FEATURES="-ipc-sandbox -network-sandbox -pid-sandbox" \
     MAKEOPTS="-j4" \
-    USE="-perl"
+    USE="-perl -ipc -rsync-verify"
 
 # https://github.com/gentoo/gentoo-docker-images/issues/79
 RUN emerge --unmerge dev-python/setuptools dev-python/certifi \
  && emerge dev-vcs/git app-portage/repoman dev-util/pkgcheck
+ && rm -rf /var/cache/distfiles/*