Basic readme for keycloak container/image. [skip-ci]

ops/containers/keycloak/README.md

+This basically uses the "official" 7.0.0 keycloak with some additions
+
+- material theme
+- bcrypt support
+- a custom form action to check the username (that has to be build before building the image!)
+- realm definitions for prod and test realm
+- scripts for import and export of all users
+
+To build the image:
+```
+cd registration_form_action
+mvn package
+cd ..
+docker build -t nomad/keycloak .
+```
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