gitlab-ci: Fix source tarball job

* Use show! in source tarball job. Since we aren't actually building
  anything in this job `show` won't
  work.

* Fix Docker image name

* Make `version` file contain only version string

.gitlab-ci.yml

@@ -701,7 +701,9 @@ source-tarball:
     - ./configure
     - make sdist
     - mv sdistprep/*.xz  .
-    - make show VALUE=version > version
+    - make show! --quiet VALUE=ProjectVersion > version
+    - source version
+    - echo "$ProjectVersion" > version
 
 
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