Add Dockerfile and adapt CI config

.gitlab-ci.yml

-image: node:10
+image: Jailbreak-Paris/validata-doc:latest
 
 before_script:
-  - apt --yes update
-  - apt --yes install --no-install-recommends calibre rsync
-  # image node:10 already provides openssh-client
-
-  - npm install gitbook-cli -g
-
   # Run ssh-agent (inside the build environment).
   - eval $(ssh-agent -s)
 
@@ -22,6 +16,5 @@ job:
   variables:
     PDF_FILE: Socle Commun des Données Locales.pdf
   script:
-    - gitbook install
     - ./build.sh
     - rsync -avz --delete _book/ "$PDF_FILE" validata@go.validata.fr:validata-doc-html/

Dockerfile

+FROM node:10
+
+MAINTAINER Validata Team
+
+RUN apt --yes update
+RUN apt --yes install --no-install-recommends calibre rsync
+# The image "node:10" already provides "openssh-client".
+
+RUN npm install gitbook-cli -g
+RUN gitbook install
+
+# From https://github.com/docker-library/python/blob/master/Dockerfile-debian.template
+ENV LANG C.UTF-8