Rename 'android' docker image back to 'ubuntu'

There is no point in doing a different docker image for each build we
are going to do inside an Ubuntu distro. We can later use the same image
for native ubuntu builds, or other cross builds (e.g. android API 21,
etc).

.gitlab-ci.yml

@@ -92,14 +92,13 @@ fedora amd64 docker:
     DOCKERFILE: "docker/fedora/Dockerfile"
   extends: .base
 
-# Cross compile image
-android 28 amd64 docker:
+ubuntu amd64 docker:
   stage: "build docker"
   variables:
     ARCH: "amd64"
-    TAG: "android_28"
-    CONTEXT_DIR: "docker/android/"
-    DOCKERFILE: "docker/android/Dockerfile"
+    TAG: "ubuntu"
+    CONTEXT_DIR: "docker/ubuntu/"
+    DOCKERFILE: "docker/ubuntu/Dockerfile"
   extends: .base
 
 cerbero fedora amd64 docker: