[build] docker: add entrypoint script

First of all, this script moves the duniter configuration file 'conf.json'
into the '/etc/duniter' volume, so that one could delete the '/var/lib/duniter'
volume without losing the configuration.

A symbolic link is set up into /var/lib/duniter/duniter_default to point to
the new path:
 /var/lib/duniter/duniter_default/conf.json -> /etc/duniter/conf.json

Then, this script enables the folowing environment variables:

* DUNITER_MANUAL_CONFIG (boolean, default = false)

  When set to 'true' (or 'yes', or '1'), the script waits until the file
  '/etc/duniter/conf.json.orig' is present before starting the duniter
  service. Here is the workflow when enabled:
  1. wait for '/etc/duniter/conf.json.orig'
  2. if this file was changed since the previous startup:
    2.1. Save 'conf.json' to 'conf.json.old'
    2.2. Replace 'conf.json' with 'conf.json.orig'
    2.3. Save the new file's checksum to compare with at next startup
  3. continue the startup sequence

  When set to false, the startup sequence continues.

  Requires 'jq' to sort the files before comparing them.

* DUNITER_AUTO_SYNC (boolean, default = false)
  Requires 'DUNITER_SYNC_HOST'.

  When set to 'true' (or 'yes', or '1') and the folder
  '/var/lib/duniter/duniter_default/data' doesn't exist, a 'duniter sync'
  command will be issued before starting the service.

  When set to false, the service is started directly.

* DUNITER_SYNC_HOST (hostname, default = "")

  This is the 'host:port' parameter to use with 'duniter sync' when
  'DUNITER_AUTO_SYNC' is enabled.

  The synchronization won't be launched when the variable is not defined
  or empty.