Update Alpine 3.10 -> 3.11; update pip, tzdata, gunicorn

Add docker secrets support

Correct request method

Add missing format arguments for error message

Corrections to Azure auth service definition after refactor

Fix token name in session for OIDC and GitHub authentication

The python float number formatting causes unit tests failed

Perform domain sync within a single transaction

Add log message after remote server request

This might be handy to detect remote server problems or debugging
timeouts and the like.

This increases data inconsistency and also performance of the whole
update process: SqlAlchemy can use its own caching better with a single
transaction as it does not need to mark already fetched objects as dirty
and so re-fetch them again on later use. So after deleting a domain,
we can still use the previously fetched domain objects for the update
operations afterwards. Otherwise SqlAlchemy had to fetch each domain
object again with single SELECT statements which leads to bad
performance with many domains (>> 1000).

The introduced "do_commit" variable controls whether the database
changes are to be committed by the caller or in the called method
itself. So we can use a single transaction for the Domain.update()
method while still preserving the previous behavior for other callers.

Closes #428.

Add new setting to verify outgoing SSL connections

Fix logging in models

Introduce PDNS_ADMIN_LOG_LEVEL to change the log level

Fiy typos in logging messages

By setting the environment variable PDNS_ADMIN_LOG_LEVEL to a Python
support log level, PowerDNS-Admin will use that log level.