Feature docker secrets

[rockaut]

should fix #458

Works with by checking in docker-entrypoint.sh if specified env variables are present with _FILE appended:

• MYSQL_USER
• MYSQL_PASSWORD
• MYSQL_ROOT_PASSWORD
• POSTGRES_USER
• POSTGRES_PASSWORD

That can be done also in .env_db_mysql and so on (see examples).

In docker swarm you then set:

echo "mysqldbuser" | docker secret create MYSQL_USER -
echo "mysqldpass" | docker secret create MYSQL_PASSWORD -
echo "mysqldroot" | docker secret create MYSQL_ROOT_PASSWORD -
echo "postgresuser" | docker secret create POSTGRES_USER -
echo "postgrespass" | docker secret create POSTGRES_PASSWORD -

And refence the secrets in the stack services.

Tested on katacoda for all arches (ubuntu,centos,alpine) only error was like in #468 but those are also present in current zabbix/zabbix-server-pgsql:ubuntu-latest official builds for example.

[rockaut]

@dotneft currently tested and PRed against 4.2 branch.
Should i also to against others? Didn't how you accept PRs - could also do separate branches for other verions (4.0, 3.2, ...)

[rockaut]

@grumbens @dotneft can I do anything to make this PR more mergeable?

[dotneft]

Thank you! Added!