gitlab-ci: Run alpine builds during nightly job

.gitlab-ci.yml

@@ -580,7 +580,7 @@ release-x86_64-linux-deb8:
 # x86_64-linux-alpine
 #################################
 
-release-x86_64-linux-alpine:
+.build-x86_64-linux-alpine:
   extends: .validate-linux
   stage: full-build
   image: "registry.gitlab.haskell.org/ghc/ci-images/x86_64-linux-alpine:$DOCKER_REV"
@@ -592,14 +592,23 @@ release-x86_64-linux-alpine:
     BIN_DIST_PREP_TAR_COMP: "bindistprep/ghc-x86_64-alpine-linux.tar.xz"
     # Can't use ld.gold due to #13958.
     CONFIGURE_ARGS: "--disable-ld-override"
-  only:
-    - tags
   cache:
     key: linux-x86_64-alpine
   artifacts:
     when: always
     expire_in: 2 week
 
+release-x86_64-linux-alpine:
+  extends: .build-x86_64-linux-alpine
+  only:
+    - tags
+
+nightly-x86_64-linux-alpine:
+  extends: .build-x86_64-linux-alpine
+  only:
+    variables:
+      - $NIGHTLY
+
 #################################
 # x86_64-linux-centos7
 #################################