version: '2'

services:

  bugzilla:
    build: bugzilla
    image: my/bugzilla:1.0.0
    container_name: my-bugzilla
    restart: always
#    env_file:
#      - ./.env
    environment:
      MYSQL_DB: ${BUGZILLA_MARIADB_DB}
      MYSQL_USER: ${BUGZILLA_MARIADB_USER}
      MYSQL_PWD: ${BUGZILLA_MARIADB_PASSWORD}
    depends_on:
      - db
    ports:
      - 8080:80
    networks:
      bugzilla_db:
      bugzilla:
        aliases:
          - my-bugzilla

  db:
    build: mariadb
    image: my/mariadb
    container_name: my-bugzilla-db
    restart: always
    environment:
      MYSQL_ROOT_PASSWORD: ${BUGZILLA_MARIADB_PASSWORD}
    volumes:
      - data-bugzilla-db:/var/lib/mysql:z
    ports:
      - '3306:3306'
    networks:
      bugzilla_db:
        aliases:
          - my-bugzilla-db

volumes:
  data-bugzilla-db:

networks:
  bugzilla:
  bugzilla_db:

# docker network inspect dockerbugzillaupgrade_bugzilla_db
[
    {
        "Name": "dockerbugzillaupgrade_bugzilla_db",
        "Id": "1b6e7cb4c382c35cfb23b7ba247584b1469a7d9d076c6e609106ca4d535f19d1",
        "Created": "2017-10-23T16:20:48.156537994+02:00",
        "Scope": "local",
        "Driver": "bridge",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": null,
            "Config": [
                {
                    "Subnet": "172.21.0.0/16",
                    "Gateway": "172.21.0.1"
                }
            ]
        },
        "Internal": false,
        "Attachable": false,
        "Containers": {
            "0af71c056d0e995265422e66673c65438e4585f5478f3a57fa3460f9fb8c1c25": {
                "Name": "my-bugzilla",
                "EndpointID": "24efee64b6aebdbdc0e9304e7bc484e042b2b6c41b92938044ac090ac5d7020d",
                "MacAddress": "02:42:ac:15:00:03",
                "IPv4Address": "172.21.0.3/16",
                "IPv6Address": ""
            },
            "586d0ae09a894510aa892c70db038041b1d886f282224c6c9bda9b1eae68a452": {
                "Name": "my-bugzilla-db",
                "EndpointID": "8525978699f3043066b61f95939fad43c53647166fa82c723bbb451974cfa04a",
                "MacAddress": "02:42:ac:15:00:02",
                "IPv4Address": "172.21.0.2/16",
                "IPv6Address": ""
            }
        },
        "Options": {},
        "Labels": {}
    }
]

$db_host = 'my-bugzilla-db';

Reading ./localconfig...
Checking for            DBD-mysql (v4.001)    ok: found v4.043
There was an error connecting to MySQL:

    Access denied for user 'bugs'@'172.21.0.3' (using password: YES)

This might have several reasons:

* MySQL is not running.
...

sh-4.4# curl  my-bugzilla-db:3306
5.5.5-10.1.19-MariaDB
=ztU/'s4�?�E@OMu%SS'dHzmysql_native_password!��#08S01Got packets out of ordersh-4.4#