My files:

postgresql_data/data/init.table.csv

id,name,completed
1,Buy pen,false
2,Write letter,false
3,Post letter,false
4,Post letter,false

docker-compose.yml

services:
  postgres-example-pg-hba:
    image: postgres
    environment:
      - POSTGRES_DB=mydb
      - POSTGRES_USER=mysuperuser
      - POSTGRES_PASSWORD=mypassword
    build:
      context: .
    ports:
          - "5432:5432"
    restart: unless-stopped

Dockerfile

FROM postgres:latest

# copy-data.sh
COPY ./postgresql_data ./temp

# Copy init.table.csv to /var/lib/postgresql/data/init.table.csv 
#   - Wait for postgres build to have created folder: /var/lib/postgresql/data 
#   - Then on docker-entrypoint-initdb.d/copy-data.sh
COPY ./scripts/copy-data.sh /docker-entrypoint-initdb.d/copy-data.sh
# cp /temp/data/init.table.csv -u /var/lib/postgresql/data/init.table.csv 

# Initialise database with a create table
COPY ./scripts/seed.sql /docker-entrypoint-initdb.d/seed.sql
# CREATE TABLE IF NOT EXISTS mytable(
#   id SERIAL PRIMARY KEY,
#   name VARCHAR(500) NOT NULL,
#   completed BOOLEAN NOT NULL
# );
# COPY mytable FROM '/var/lib/postgresql/data/init.table.csv' DELIMITER ',' CSV HEADER;

This works with docker-compose up –build

docker-compose up -d --build
docker exec -it dockerise-postgres-postgres-example-pg-hba-1 psql -U mysuperuser -d mydb
select * from mytable;

output

 id |     name     | completed
----+--------------+-----------
  1 | Buy pen      | f
  2 | Write letter | f
  3 | Post letter  | f
  4 | Post letter  | f
(4 rows)

This does NOT work when I do docker build . first then docker-compose up

docker-compose down
docker image rm postgres

docker build .
docker-compose up
docker exec -it dockerise-postgres-postgres-example-pg-hba-1 psql -U mysuperuser -d mydb
select * from mytable;

mydb=# select * from mytable;
ERROR: relation “mytable” does not exist
LINE 1: select * from mytable;