Docker PostgreSQL change database encoding to UTF-8

I want to run via docker-compose a postgres container which has COLLATE and CTYPE 'C' and database encoding 'UTF-8'. But this looks to be impossible.

This is the part on the docker-compose.yml:

database:
    image: postgres:latest
    volumes:
        - db:/var/lib/postgresql/data
    environment:
        POSTGRES_PASSWORD: test
        LC_COLLATE: C
        LC_CTYPE: C
        LANG: C.UTF-8

And this is the log output:

The database cluster will be initialized with locales.
The default text search configuration will be set to "english".
  COLLATE:  C
  CTYPE:    C
  MESSAGES: C.UTF-8
  MONETARY: C.UTF-8
  NUMERIC:  C.UTF-8
  TIME:     C.UTF-8
The default database encoding has accordingly been set to "SQL_ASCII".

I must have the database encoding in UTF-8 and the COLLATE and CTYPE in 'C' and not 'C.UTF-8' as otherwise a dependend application cannot connect.

I didn't find anything in any documentation or anywhere else.

You can actually combine the build into docker compose:

services:
  postgres:
    container_name: postgres
    image: postgres-de
    build: ./pg-docker-config  # a Dockerfile must reside in this directory
    environment:
      - POSTGRES_USER=${POSTGRES_USER}
      - POSTGRES_PASSWORD=${POSTGRES_PW}
      - POSTGRES_DB=${POSTGRES_DB} #optional (specify default database instead of $POSTGRES_DB)
      - PGDATA=/var/lib/postgresql/data/pgdata
      - POSTGRES_INITDB_ARGS='--locale=de_DE.UTF8'
    ports:
      - "5432:5432"
    # volumes:
      # - /postgres/dev:/var/lib/postgresql/data
    restart: unless-stopped

The Dockerfile in ./pg-docker-config looks like this

FROM postgres:latest
RUN localedef -i de_DE -c -f UTF-8 -A /usr/share/locale/locale.alias de_DE.UTF-8
ENV LANG de_DE.utf8

Make sure to delete your existing database volume(s) before composing again by running ...

docker compose down --volumes

... or else you will see this message, and the database will not initialize the locale settings

PostgreSQL Database directory appears to contain a database; Skipping initialization

Now you can launch the container(s) with only one command :)

docker compose up -d

I tried the approach above, and another issue I encountered was that the default Debian base that comes with the official Postgres Docker image doesn't have any other languages installed.

# locale -a
C
C.UTF-8
en_US.utf8
POSIX

To set another language, e.g. Chinese, it was necessary to run e.g. localedef -i zh_CN -c -f UTF-8 -A /usr/share/locale/locale.alias zh_CN.UTF-8.

Then setting the environment variables worked.

Because of this need, it seems that the default Postgres image doesn't satisfy all the needs, and it would be better to build a custom image, as documented in this post.

FROM postgres
RUN localedef -i zh_CN -c -f UTF-8 -A /usr/share/locale/locale.alias zh_CN.UTF-8
ENV LANG zh_CN.utf8

Then docker build -t your-custom-image-name .

Then you can use the custom image in your docker-compose.yml instead of the official postgres image, without needing to set any environment variable in addition.