The Dockerfiles in our doc- repos are confusing

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Having Dockerfiles in our doc- repos gives people the interesting idea that they should be able to easily build documentation using them. Which isn't really the case. Since they're only a few lines long, maybe we could write them via .travis.yml instead.

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Nicolas Bock has rewritten the SOC stuff. However, as he's used an actual Bash script instead of the .travis.yml file, we'd need to check that his stuff translates 1:1:

-  - ls
-  - echo "Building docker image"
-  - docker build -t sle-doc-image .

 script:
-  - docker run --rm -it sle-doc-image /bin/bash -c '/bin/bash travis.sh'
+  - docker run --rm -it --volume ${PWD}:/usr/src/app --workdir /usr/src/app susedoc/ci:openSUSE-42.3 /bin/bash -c '/bin/bash travis.sh'

actual file

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This is done across repos and branches.