Can't connect to PostgreSQL database in Docker container

Question

I created web application using Python 3.7, Flask and PostgreSQL. To deploy it, I built the Docker container and tried to run it, but got the following error:

D:\PyCharms\Module 6\app>docker run --env DB_URL=postgresql+psycopg2://test_user:password@127.0.0.1:5432/test_db -p 8000:8000 doc
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 2336, in _wrap_pool_connect
    return fn()
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 364, in connect
    return _ConnectionFairy._checkout(self)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 778, in _checkout
    fairy = _ConnectionRecord.checkout(pool)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 495, in checkout
    rec = pool._do_get()
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/impl.py", line 140, in _do_get
    self._dec_overflow()
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/langhelpers.py", line 70, in __exit__
    with_traceback=exc_tb,
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/compat.py", line 182, in raise_
    raise exception
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/impl.py", line 137, in _do_get
    return self._create_connection()
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 309, in _create_connection
    return _ConnectionRecord(self)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 440, in __init__
    self.__connect(first_connect_check=True)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 661, in __connect
    pool.logger.debug("Error on connect(): %s", e)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/langhelpers.py", line 70, in __exit__
    with_traceback=exc_tb,
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/compat.py", line 182, in raise_
    raise exception
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 656, in __connect
    connection = pool._invoke_creator(self)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/strategies.py", line 114, in connect
    return dialect.connect(*cargs, **cparams)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/default.py", line 493, in connect
    return self.dbapi.connect(*cargs, **cparams)
  File "/usr/local/lib/python3.7/site-packages/psycopg2/__init__.py", line 127, in connect
    conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
psycopg2.OperationalError: could not connect to server: Connection refused
        Is the server running on host "127.0.0.1" and accepting
        TCP/IP connections on port 5432?
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
  File "./run.py", line 3, in <module>
    app = create_app()
  File "/app/core/__init__.py", line 21, in create_app
    db.create_all()
  File "/usr/local/lib/python3.7/site-packages/flask_sqlalchemy/__init__.py", line 1039, in create_all
    self._execute_for_all_tables(app, bind, 'create_all')
  File "/usr/local/lib/python3.7/site-packages/flask_sqlalchemy/__init__.py", line 1031, in _execute_for_all_tables
    op(bind=self.get_engine(app, bind), **extra)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/sql/schema.py", line 4554, in create_all
    ddl.SchemaGenerator, self, checkfirst=checkfirst, tables=tables
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 2094, in _run_visitor
    with self._optional_conn_ctx_manager(connection) as conn:
  File "/usr/local/lib/python3.7/contextlib.py", line 112, in __enter__
    return next(self.gen)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 2086, in _optional_conn_ctx_manager
    with self._contextual_connect() as conn:
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 2302, in _contextual_connect
    self._wrap_pool_connect(self.pool.connect, None),
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 2340, in _wrap_pool_connect
    e, dialect, self
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 1584, in _handle_dbapi_exception_noconnection
    sqlalchemy_exception, with_traceback=exc_info[2], from_=e
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/compat.py", line 182, in raise_
    raise exception
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 2336, in _wrap_pool_connect
    return fn()
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 364, in connect
    return _ConnectionFairy._checkout(self)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 778, in _checkout
    fairy = _ConnectionRecord.checkout(pool)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 495, in checkout
    rec = pool._do_get()
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/impl.py", line 140, in _do_get
    self._dec_overflow()
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/langhelpers.py", line 70, in __exit__
    with_traceback=exc_tb,
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/compat.py", line 182, in raise_
    raise exception
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/impl.py", line 137, in _do_get
    return self._create_connection()
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 309, in _create_connection
    return _ConnectionRecord(self)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 440, in __init__
    self.__connect(first_connect_check=True)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 661, in __connect
    pool.logger.debug("Error on connect(): %s", e)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/langhelpers.py", line 70, in __exit__
    with_traceback=exc_tb,
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/compat.py", line 182, in raise_
    raise exception
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 656, in __connect
    connection = pool._invoke_creator(self)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/strategies.py", line 114, in connect
    return dialect.connect(*cargs, **cparams)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/default.py", line 493, in connect
    return self.dbapi.connect(*cargs, **cparams)
  File "/usr/local/lib/python3.7/site-packages/psycopg2/__init__.py", line 127, in connect
    conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
sqlalchemy.exc.OperationalError: (psycopg2.OperationalError) could not connect to server: Connection refused
        Is the server running on host "127.0.0.1" and accepting
        TCP/IP connections on port 5432?

DB_URL is an environment variable used for setting up Postgre database. On my local computer everything seems to work fine. What am I doing wrong?

UPD: Here is my Dockerfile.

FROM python:3.7-alpine3.7

WORKDIR /app

COPY requirements.txt ./
RUN \
 apk add --no-cache postgresql-libs && \
 apk add --no-cache --virtual .build-deps gcc musl-dev postgresql-dev && \
 python3 -m pip install -r requirements.txt --no-cache-dir && \
 apk --purge del .build-deps

RUN export PYTHONPATH='${PYTHONPATH}:/app'

COPY . .

CMD ["python", "./run.py"]

Answer 1

This might help. If not, you may need to add a rule to your firewall to permit connections from docker0. On Linux, this is:

# iptables -A INPUT -i docker0 -j ACCEPT

OSX and windows have an interface to access their firewall rules. Although it is possible to modify their rules using command line, they are known to cause bugs in the display interface.