build.sh: More convenient way to push images

Since build.sh now builds more than just two images, individual queries
whether to push each image to a registry may become tedious.  This patch
replaces the queries with a single checklist menu.  After making
a selection, the images get pushed all at once.

In the menu, all images are unchecked by default in order to prevent
accidental uploads.  This, too, may become tedious.  In that case, the
default could be flipped or a new option could be introduced.

docker/build.sh

@@ -91,13 +91,22 @@ for i in "${SELECTED_TARGETS[@]}"; do
   else
     docker build --tag "$img" --pull "${OPTIONS[@]}" "$loc"
   fi
-  BUILT_IMAGES+=("$img")
+  BUILT_IMAGES+=("$img OFF")
 done
 
-for img in "${BUILT_IMAGES[@]}"; do
-  read -p "Push image '$img' to repository? [y/N] " REPL
-  case "$REPL" in
-    Y|y|Yes|yes|YES)
-      docker push "$img" ;;
-  esac
-done
+if hash whiptail > /dev/null 2>&1; then
+  while read img; do
+    echo "Pushing ${img}."
+    docker push "$img"
+  done < <( whiptail --title "OpenSlides build script" \
+    --checklist "Select images to push to their registry." \
+    25 78 16 --separate-output --noitem --clear \
+    ${BUILT_IMAGES[@]} \
+    3>&2 2>&1 1>&3 )
+else
+  printf "\nSuccessfully built images:\n\n"
+  for i in "${BUILT_IMAGES[@]}"; do
+    read -r img x <<< "$i"
+    printf "  - $img\n"
+  done
+fi