feat(bma): configure with env var

See merge request nodes/typescript/duniter!1382

Docker: move config back to volume /var/lib/duniter and other minor changes

See merge request nodes/typescript/duniter!1381

Moving `conf.json` from volume `/var/lib/duniter` to volume `/etc/duniter`
was a mistake. It creates to much many corner cases where one can lose
their configuration file.

This commit partly reverts fd0bbe67 by bringing conf.json back into volume
`/var/lib/duniter`.

Was /duniter/duniter.

Spare one process.

And link duniter binary as /usr/bin/duniter.

little closer to conventionalcommits.org

[ref] externalyze gva configuration

See merge request nodes/typescript/duniter!1379

Docker entrypoint - fixes and error handling

See merge request nodes/typescript/duniter!1380

When `/etc/duniter` is not mounted as a persistent volume, the
configuration file is lost after re-creating the instance.

To prevent this, before transitionning the configuration file from
volume '/var/lib/duniter' to volume '/etc/duniter', the entrypoint
backups it into '/var/lib/duniter'.  On restart, if the entrypoint
detects that the link to '/etc/duniter/conf.json' is dangling, it
prints a big fat warning into the logs and re-use the backuped
config file.

Boolean variable to enable `set -x` in the entrypoint script.

The script '/usr/bin/duniter' sets this option as well and the duniter
executable won't accept more than one.

And uset `set -u` to prevent this from happening again.

This reverts commit 59ad1347.