Fix mariadb issue

[dpordomingo]

Install libmariadb3 on image to avoid

ERROR [root] libmariadbclient.so.18: cannot open shared object file: No such file or directory

as reported by slack:
if I do:

$ GO111MODULE=on go install ./cmd/sourced
$ sourced version
sourced version master build dev
$ sourced compose version
$ docker rmi srcd/sourced-ui :v0.5.0
$ VERSION=v0.5.0 make build
$ sourced init local .

and check source{d} CE status, I see that sourced-ui is in a restart loop, logging this:

ERROR [root] libmariadbclient.so.18: cannot open shared object file: No such file or directory
Traceback (most recent call last):
  File "/home/superset/superset/models/core.py", line 1092, in get_all_table_names_in_schema
    inspector=self.inspector, schema=schema
  File "/home/superset/superset/models/core.py", line 1040, in inspector
    engine = self.get_sqla_engine()
  File "/home/superset/superset/utils/core.py", line 134, in __call__
    value = self.func(*args, **kwargs)
  File "/home/superset/superset/models/core.py", line 937, in get_sqla_engine
    return create_engine(url, **params)
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/engine/__init__.py", line 435, in create_engine
    return strategy.create(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/engine/strategies.py", line 87, in create
    dbapi = dialect_cls.dbapi(**dbapi_args)
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/dialects/mysql/mysqldb.py", line 118, in dbapi
    return __import__("MySQLdb")
  File "/usr/local/lib/python3.6/site-packages/MySQLdb/__init__.py", line 18, in <module>
    from . import _mysql
ImportError: libmariadbclient.so.18: cannot open shared object file: No such file or directory
Loaded your LOCAL configuration at [/home/superset/superset/superset_config.py]
Traceback (most recent call last):
  File "bootstrap.py", line 122, in <module>
    bootstrap()
  File "bootstrap.py", line 79, in bootstrap
    create_datasource_tables(dbobj, conf.get('GITBASE_DB'))
  File "bootstrap.py", line 30, in create_datasource_tables
    for ds in dbobj.get_all_table_names_in_schema(schema):
TypeError: 'NoneType' object is not iterable

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• I have updated the CHANGELOG file according to the conventions in keepachangelog.com
• This PR contains changes that do not require a mention in the CHANGELOG file