Add docker-ensure-installed.sh as an explicit entrypoint/symlink

This allows using the image as `docker run --mount ... wordpress docker-ensure-installed.sh` which will then *just* ensure WordPress is installed in the volume and exit (successfully).

The primary use case for this is an `initContainer` in Kubernetes.

Author: tianon

Dockerfile.template

@@ -257,6 +257,10 @@ VOLUME /var/www/html
 COPY --chown=www-data:www-data wp-config-docker.php /usr/src/wordpress/
 {{ ) else "" end -}}
 COPY docker-entrypoint.sh /usr/local/bin/
+{{ if env.version != "cli" then ( -}}
+# https://github.com/docker-library/wordpress/issues/969
+RUN ln -svfT docker-entrypoint.sh /usr/local/bin/docker-ensure-installed.sh
+{{ ) else "" end -}}
 
 ENTRYPOINT ["docker-entrypoint.sh"]
 {{ if env.version != "cli" then ( -}}