feat: env vars for cpu and nbCores pow parameters

See merge request !1405

DUNITER_POW_CPU
DUNITER_POW_NBCORES

ci: Use extends

See merge request !1404

fix(docker): typo in key filename

See merge request !1403

Using '/etc/duniter/keyring.yml' to be consistent with the default
filename '/var/lib/duniter/duniter_default/keyring.yml'.

feat(conf): add ability to configure pow prefix with env var

See merge request !1402

Resolve "Duniter listens to [::]:8888 even if not asked by BMA/GVA/WS2P"

Closes #1431

See merge request !1401

Resolve "`sync` uses too much RAM, even without GVA enabled"

Closes #1430

See merge request !1400

feat(docker): build and install dex

See merge request !1397

deps: include gva module in x86_64 arch only

See merge request !1398