Postgres installation on Docker Ruby Alpine - ruby

Question

I'm trying to complete the setup for my Rails application, using Docker. For development I use docker-compose.yml, and for production just the Dockerfile, as I want to deploy using Dokku (which has no support for compose).
In development everything is running fine, I'm getting my containers (app, webpacker and postgres), but in production mode my postgres container is missing... and also Dokku can't deploy correctly. I tried many ways, but nothing seems to work :(
Dockerfile
FROM ruby:2.5.5-alpine
ARG PRECOMPILEASSETS
ENV NODE_OPTIONS "--max_old_space_size=4096"
ENV SECRET_KEY_BASE=foo
RUN apk add --update --no-cache \
build-base \
git \
postgresql-dev \
postgresql-client \
imagemagick \
nodejs-current \
yarn \
python2 \
tzdata \
file
RUN gem install bundler
# Install gems
RUN mkdir /gems
WORKDIR /gems
COPY Gemfile .
COPY Gemfile.lock .
RUN bundle install -j4 --retry 3 \
# Remove unneeded files (cached *.gem, *.o, *.c)
&& rm -rf /usr/local/bundle/cache/*.gem \
&& find /usr/local/bundle/gems/ -name "*.c" -delete \
&& find /usr/local/bundle/gems/ -name "*.o" -delete
RUN yarn install
ARG INSTALL_PATH=/beweeg
ENV INSTALL_PATH $INSTALL_PATH
WORKDIR $INSTALL_PATH
COPY . .
# Precompile assets (or not)
RUN docker/potential_asset_precompile.sh $PRECOMPILEASSETS
# Expose Puma port
EXPOSE 3000
CMD ["docker/startup.sh"]
docker-compose.yml
version: '3.0'
services:
db:
image: postgres:11-alpine
ports:
- 5433:5432
environment:
POSTGRES_PASSWORD: postgres
webpacker:
image: beweeg_development
command: bin/webpack-dev-server
volumes:
- .:/beweeg:cached
ports:
- 3035:3035
app:
image: beweeg_development
build:
context: .
args:
- PRECOMPILEASSETS=NO
links:
- db
- webpacker
ports:
- 3000:3000
volumes:
- .:/beweeg:cached
As you'll see using docker-compose makes no problem, but if I want to build and run as production (without compose), then I cannot get pg installed correctly.
Previously I was using a Ruby slim image, and then using the following command:
RUN apt-get update && apt-get install -y curl gnupg
RUN echo "deb http://apt.postgresql.org/pub/repos/apt/ stretch-pgdg main" > /etc/apt/sources.list.d/pgdg.list
RUN curl -q https://www.postgresql.org/media/keys/ACCC4CF8.asc | apt-key add -
was working nicely... but in order to reduce my images sizes, I'd prefer to stay now with Alpine.
I'm quite a noob, so please forgive me if the answer seems obvious... And thanks in advance for your help!