Assign the image tag on its own line in script/ (closes #101)

script/cibuild, script/docker, script/lint and script/test passed the
tag from script/projectname inline to docker build, where a failing
command substitution does not trip set -e. Each now assigns it to `tag`
on its own line first, so the script stops where script/projectname
fails. The comment above it says why; the REPO_POLICIES.md snippets show
the same form.

Model: opus-5-5

script/test

@@ -11,9 +11,13 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
+    # The tag gets its own line: a failing command substitution inside
+    # an argument does not trip `set -e`, so the inline form degrades
+    # silently to an empty constant.
+    tag="$("$SCRIPT_DIR/projectname")"
     docker build --no-cache \
         --target test \
-        -t "$("$SCRIPT_DIR/projectname")-test" .
+        -t "$tag-test" .
 }
 
 main "$@"