docs: how to update mediawiki wiki.pub.solar

docs/mediawiki-updates.md

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+# Updating mediawiki docker container
+
+See the [mediawiki-oidc-docker repository](https://git.pub.solar/pub-solar/mediawiki-oidc-docker#updating-the-docker-image)
+for instructions on updating our customized mediawiki docker image.
+
+To deploy a new docker image to `nachtigall`, first bump the mediawiki version
+of the docker image tag in `hosts/nachtigall/apps/mediawiki.nix` (search for
+`image`).
+
+Next, push your changes to https://git.pub.solar and get them reviewed and
+approved.
+
+After approval, create a fresh backup of the database and deploy the changes to
+`nachtigall`:
+
+```
+ssh barkeeper@nachtigall.pub.solar
+sudo -u postgres pg_dump --create -Fc mediawiki > mediawiki-db-$(date +%F).dump
+exit
+```
+
+```
+deploy --targets '.#nachtigall'
+```
+
+Then, finalize the update by running the database migration script:
+```
+ssh barkeeper@nachtigall.pub.solar
+docker exec -it mediawiki bash
+php maintenance/run.php update.php
+```