Build and use the new docker image with cleveref (modelica#2626)

Also added a README for updating the docker image

.CI/Jenkinsfile

@@ -18,7 +18,7 @@ pipeline {
   stage('build') {
     agent {
       docker {
-        image 'modelicaspec/latexml:20200415'
+        image 'modelicaspec/latexml:20200803'
         label 'linux'
         alwaysPull true
       }

.CI/latexml/Dockerfile.incremental

@@ -0,0 +1,5 @@
+FROM modelicaspec/latexml:20200415
+
+RUN tlmgr update --self \
+ && tlmgr install cleveref \
+ && apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/ /root/.cpanm/work*

.CI/latexml/README.md

@@ -0,0 +1,24 @@
+# Updating the docker image
+
+The docker image is stored at the [docker hub](https://hub.docker.com/r/modelicaspec/latexml).
+If you need write access to the organization, contact @sjoelund.
+
+To update the image, update the Dockerfile .
+Then build and tag the image and upload it.
+
+```sh
+docker build -t modelicaspec/latexml:`date +%Y%m%d` .
+docker push modelicaspec/latexml:`date +%Y%m%d`
+```
+
+If the change is small you can base it on the old image instead of creating a new one from scratch.
+Modify Dockerfile.incremental to be based on the image you want to update.
+Make sure that the regular Dockerfile is also updated (so future builds incorporates these additions).
+
+```sh
+docker build -t modelicaspec/latexml:`date +%Y%m%d` - < Dockerfile.incremental
+docker push modelicaspec/latexml:`date +%Y%m%d`
+```
+
+Once the docker image is updated, modify `../Jenkinsfile` to use this image.
+Put all of this in the same commit and see if the CI build accepts it.