Docker and PostgreSQL – Password_File Leads to No pg_hba.conf Entry

I'm aware of the following solution for the no pg_hba.conf entry for host error: connect to PostgreSQL server: FATAL: no pg_hba.conf entry for host

But I wonder why this error happens when using Postgres environment variable POSTGRES_PASSWORD_FILE in Docker, and not when setting the password directly with an environment variable such as POSTGRES_PASSWORD.

Here are the most useful bits of code. If you want to see a complete example, please take a look at this repository.

This version will trigger the error:

services:
  database:
    image: postgres:10
    volumes:
      - ./pass/db/db_pass:/run/secrets/db_pass:ro
      - data:/var/lib/postgresql/data
    environment:
      - POSTGRES_PASSWORD_FILE=/run/secrets/db_pass

While this version will work fine:

services:
  database:
    image: postgres:10
    volumes:
      - data:/var/lib/postgresql/data
    environment:
      - POSTGRES_PASSWORD=db_password

The ./pass/db/db_pass file contains exactly the same password: db_password.

Basically, the only thing that changes between the two configuration is that setting the password for the database container is 1) done with an environment variable POSTGRES_PASSWORD and 2) done reading the file pointed by POSTGRES_PASSWORD_FILE, file which is bind-mounted inside the container as read-only.

If we look at the Postgres' entrypoint, POSTGRES_PASSWORD_FILE is used to fill POSTGRES_PASSWORD: https://github.com/docker-library/postgres/blob/master/10/docker-entrypoint.sh#L8-L24.

# usage: file_env VAR [DEFAULT]
#    ie: file_env 'XYZ_DB_PASSWORD' 'example'
# (will allow for "$XYZ_DB_PASSWORD_FILE" to fill in the value of
#  "$XYZ_DB_PASSWORD" from a file, especially for Docker's secrets feature)
file_env() {
    local var="$1"
    local fileVar="${var}_FILE"
    local def="${2:-}"
    if [ "${!var:-}" ] && [ "${!fileVar:-}" ]; then
        echo >&2 "error: both $var and $fileVar are set (but are exclusive)"
        exit 1
    fi
    local val="$def"
    if [ "${!var:-}" ]; then
        val="${!var}"
    elif [ "${!fileVar:-}" ]; then
        val="$(< "${!fileVar}")"
    fi
    export "$var"="$val"
    unset "$fileVar"
}

I don't get why then Postgres throws this no pg_hba.conf entry for host error if the database has been correctly created with the given password.

Looking at the entry point, the configuration should be the exact same, but I verified both with the command docker-compose -f no-file.yml run --rm database /bin/bash -c 'cat $PGDATA/pg_hba.conf' and the line host all all all md5 does not appear in the configuration using POSTGRES_PASSWORD_FILE.

Why does the default configuration changes when we make use of the POSTGRES_PASSWORD_FILE variable, and how to fix this elegantly? Do I have to provide a custom Postgres configuration file as described in https://hub.docker.com/_/postgres/, section "Database configuration"?