[build] docker: improve / fix entrypoint

Before transitionning the configuration file from volume '/var/lib/duniter'
to volume '/etc/duniter', the entrypoint stores a backup into the former.
This avoid losing the configuration file when '/etc/duniter' is not mounted
as a persitent volume.

When the entrypoint detects that the link to '/etc/duniter/conf.json' is
dangling, it prints a big fat warning into the log, and re-use the backuped
config file.

This commit also remove the extraneous '--home' option in the sync step.

and add 'set -ux' to prevent this happening again.
7 jobs for !1378 with pini-2 in 21 minutes and 36 seconds (queued for 21 seconds)
detached
Status Job ID Name Coverage
  Tests
passed #52568
redshift
tests

00:16:32

69.48%
 
  Package
manual #52570
redshift allowed to fail manual
package:test:docker-test-image
manual #52569
redshift allowed to fail manual
releases:test
 
  Quality
passed #52572
redshift
audit_dependencies

00:01:19

passed #52571
redshift
fmt_and_clippy

00:05:04

 
  Integration
manual #52573
redshift allowed to fail manual
sync_g1
manual #52574
redshift allowed to fail manual
sync_gtest