Docker build unable to install pg gem in Sinatra project

Question

I've reviewed all the posts on SO with similar errors installing the Postgres pg gem.

My issue is unique: I can successfully add Postgres to my Sinatra Ruby project via the Gemfile and bundle install -- however when building the Docker image it fails installing pg with the following error:

ERROR:  Error installing pg:
ERROR: Failed to build gem native extension.

current directory: /usr/local/bundle/gems/pg-0.19.0/ext
/usr/local/bin/ruby -r ./siteconf20170113-7-ottv1k.rb extconf.rb
checking for pg_config... no
No pg_config... trying anyway. If building fails, please try again with
  --with-pg-config=/path/to/pg_config
checking for libpq-fe.h... no
Can't find the 'libpq-fe.h header
*** extconf.rb failed ***

I've tried everything suggested in similar answers:

• Brew install / uninstall,
• gem install / uninstall specifying path to both files: pg_config and libpq-fe.h
• etc.

As I'm not too familiar with Docker, is there something I'm missing about installing a db with Docker?

Any help much much appreciated in advance. Thank you.

Answer 1

You got this error because of missing libpq-dev package.

Here is the minimal ubuntu-based Dockerfile for installing pg gem:

FROM ruby:2.6.6-slim-buster

RUN apt update && apt install -y \
        build-essential libpq-dev  \
     && rm -rf /var/lib/apt/lists/*

RUN gem install pg