new: doc: [rocketchat] added some update docs.

Signed-off-by: Valentin Lab <valentin.lab@kalysto.org>

rocketchat/README.org

@@ -0,0 +1,81 @@
+# -*- ispell-local-dictionary: "english" -*-
+#+SETUPFILE: ~/.emacs.d/etc/setup/latex.setup
+#+SETUPFILE: ~/.emacs.d/etc/setup/html-readtheorg-local.setup
+
+#+TITLE: Rocket.Chat
+
+* Updating the charm to a new version
+
+We are using official image. Latest tags usually. You can
+double-check available candidate for official images like this:
+
+#+begin_src sh
+docker search rocket.chat
+#+end_src
+
+So we usually pull and test images like this :
+
+#+begin_src sh
+docker pull rocket.chat:X.Y.Z
+#+end_src
+
+This worked on 3.6.3, but changed before 3.8.0 to:
+
+#+begin_src sh
+docker pull rocketchat/rocket.chat:X.Y.Z
+#+end_src
+
+from: https://hub.docker.com/r/rocketchat/rocket.chat
+
+Get the available tags:
+
+#+begin_src sh
+IMAGE_BASE_NAME=rocket.chat
+
+## lookup on docker hub only. Could think doing it for any docker
+## image identifier available on `pull`. This would mean querying
+## other registries, probably with a different api.
+docker:lookup_available_tags() {
+    local image="$1" limit="${2:-10}" filter="$3" eor p
+
+    ## remove any tags on image name
+    image="${image%%:*}"
+
+    if [[ "$image" == *"/"* ]]; then
+        username="${image%%/*}"
+        image="${image##*/}"
+    else
+        username="library"
+    fi
+
+    p=0
+    eor=
+    while [ -z "$eor" ]; do
+       ((p++))
+       out=$(curl -s "https://registry.hub.docker.com/v2/repositories/$username/$image/tags/?page=$p" |
+           jq -r '."results"[]["name"]') || break
+       [ -z "$out" ] && break
+       ## 10 results are expected per request, otherwise
+       ## this means we are hitting End of Records.
+       nb_result=$(printf "%s\n" "$out" | wc -l)
+       [ "$nb_result" -lt 10 ] && eor=true
+       ## filter records
+       [ "$filter" ] && out=$(printf "%s\n" "$out" | egrep "$filter")
+       [ -z "$out" ] && continue
+       nb_result=$(printf "%s\n" "$out" | wc -l)
+       if [ "$limit" -le "$nb_result" ]; then
+           printf "%s\n" "$out" | head -n "$limit"
+           break
+       fi
+       printf "%s\n" "$out"
+       limit=$((limit - nb_result))
+    done
+
+}
+
+docker:lookup_available_tags rocketchat/rocket.chat 15 "^[0-9]+(\.[0-9]+)*$"
+
+#+end_src
+
+Rocket.chat has a powerfull and working update database that will
+take care of migrating database on startup.