docs: add how to update mediawiki image

README.md

@@ -1,11 +1,53 @@
 # Mediawiki docker image with OIDC extension
 
+This image is based on the community maintained [mediawiki docker image](https://hub.docker.com/_/mediawiki).
+
+Currently used mediawiki extensions in this customized docker image:
+
+* [PluggableAuth](https://www.mediawiki.org/wiki/Extension:PluggableAuth)
+* [OpenID_Connect](https://www.mediawiki.org/wiki/Extension:OpenID_Connect)
+* [TemplateStyles](https://www.mediawiki.org/wiki/Extension:TemplateStyles)
+
 ### Updating the docker image
+
+Check for [new releases of mediawiki](https://www.mediawiki.org/wiki/Download)
+and take a moment to read the release notes. For a good starting point, read the
+[upgrading manual](https://www.mediawiki.org/wiki/Manual:Upgrading)
+though we use a very customized docker image and all steps necessary for
+upgrading will be mentioned here. You should also check if the newest release
+is already [published as a docker image](https://hub.docker.com/_/mediawiki)
+by the community.
+
+First, update the `FROM` image tag in the `Dockerfile` to the newest version,
+for example `1.41.0`.
+
+Next, download the newest version of each extension (matching the mediawiki docker
+image version, e.g. `1.41.0`) and get the SHA256 hash for each file:
+
+1. PluggableAuth download https://www.mediawiki.org/wiki/Special:ExtensionDistributor/PluggableAuth
+2. OpenID_Connect download https://www.mediawiki.org/wiki/Special:ExtensionDistributor/OpenIDConnect
+3. TemplateStyles download https://www.mediawiki.org/wiki/Special:ExtensionDistributor/TemplateStyles
+
+```
+NEW_PLUGGABLE_AUTH_HASH=$(sha256sum ~/Downloads/PluggableAuth-REL1_41-0273c84.tar.gz | cut -d " " -f1)
+sed -i "s/PLUGGABLE_AUTH_SHA256SUM= *[^ ]*/PLUGGABLE_AUTH_SHA256SUM=${NEW_PLUGGABLE_AUTH_HASH}/" Dockerfile
+
+NEW_OPENID_CONNECT_HASH=$(sha256sum ~/Downloads/OpenIDConnect-REL1_41-7aa039e.tar.gz | cut -d " " -f1)
+sed -i "s/OPENID_CONNECT_SHA256SUM= *[^ ]*/OPENID_CONNECT_SHA256SUM=${NEW_OPENID_CONNECT_HASH}/" Dockerfile
+
+NEW_TEMPLATE_STYLE_HASH=$(sha256sum ~/Downloads/TemplateStyles-REL1_41-a9dde29.tar.gz | cut -d " " -f1)
+sed -i "s/TEMPLATE_STYLES_SHA256SUM= *[^ ]*/TEMPLATE_STYLES_SHA256SUM=${NEW_TEMPLATE_STYLE_HASH}/" Dockerfile
+```
+
+Copy the hash in the output of each command and paste it into the variables
+`*_SHA256SUM` in `Dockerfile`, replacing the old hashes.
+
 After making your changes and [testing them locally](#local-testing), run the
 following to trigger a [CI build and push](https://git.pub.solar/pub-solar/mediawiki-oidc-docker/actions)
 of the [docker image](https://git.pub.solar/pub-solar/mediawiki-oidc-docker/packages).
+
 ```
-git add -u
+git add --update
 git commit
 git push
 ```
@@ -14,9 +56,7 @@ git push
 Build the docker image locally to test your changes.
 ```
 docker build --tag git.pub.solar/pub-solar/mediawiki-oidc-docker:test .
+docker run --detach --rm --publish 8080:80 git.pub.solar/pub-solar/mediawiki-oidc-docker:test
+
+# open http://localhost:8080 in your browser to test if mediawiki starts fine
 ```
-
-
-### Links
-- https://github.com/wikimedia/mediawiki-docker
-- https://www.mediawiki.org/wiki/Extension:OpenID_Connect