Dockerfile for using Rails 6 with Fly plus a Tailscale tunnel, with comments. Put fly_start.sh in your bin directory within the Rails app.

Dockerfile

# Heavily based off of https://github.com/fly-apps/rails-nix/blob/main/Dockerfile
# I have replaced the image from Fly's above file with rubylang's default one
# so that I know it has compatibility across platforms.

ARG RUBY_VERSION=2.6.8
FROM rubylang/ruby:${RUBY_VERSION}-bionic as patio11-build

# Copied from https://github.com/fly-apps/rails-nix/blob/main/Dockerfile

ENV PATH $PATH:/usr/local/bin

# Since Rails 6 requires some JS, installing Node, NPM, and Yarn.
# Installs a specified version of Node to overwrite Ubuntu's.

RUN --mount=type=cache,id=apt-cache,sharing=locked,target=/var/cache/apt \
    --mount=type=cache,id=apt-lib,sharing=locked,target=/var/lib/apt \
    apt-get update -qq && \
    apt-get install --no-install-recommends -y \
      git build-essential libpq-dev wget vim curl gzip xz-utils \
      libpq-dev nodejs npm && \
    rm -rf /var/lib/apt/lists /var/cache/apt/archives && \
    npm uninstall yarn -g && \
    npm install yarn -g -y && \
    npm install -g n && n 16.14.2 

# Totally optional but I like Tailscale
# Building tailscale for Ubuntu bionic using commands from https://pkgs.tailscale.com/stable/#ubuntu-bionic
# Removes the apt lists/archives at end to minimize image size/drift.
# Note that I've removed sudo from the commands because we're running as root.

FROM ubuntu:bionic as tailscale
WORKDIR /tailscale
ENV TSFILE=tailscale_1.22.2_amd64.tgz

RUN --mount=type=cache,id=apt-cache,sharing=locked,target=/var/cache/apt \
    --mount=type=cache,id=apt-lib,sharing=locked,target=/var/lib/apt \
    apt-get update -qq && \
    apt-get install --no-install-recommends -y \
      wget ca-certificates

RUN wget https://pkgs.tailscale.com/stable/${TSFILE} && tar xzf ${TSFILE} --strip-components=1

# Bundle install

FROM patio11-build AS patio11-build-rails-app

RUN mkdir /app
WORKDIR /app

ENV BUNDLE_WITHOUT development:test
ENV BUNDLE_PATH vendor/bundle

RUN gem install -N bundler -v 1.17.2

COPY Gemfile /app/Gemfile
COPY Gemfile.lock /app/Gemfile.lock

RUN bundle install

COPY . /app

# There will be a Docker checkpoint made here (among other places), which will
# keep the image relatively slim, prior to the following command pulling down
# the entire world from npm.

WORKDIR /app
ENV BUNDLE_PATH vendor/bundle

RUN bundle exec rails assets:precompile

RUN rm -rf vendor/bundle/ruby/*/cache

FROM patio11-build-rails-app

ARG RAILS_ENV=production
ENV RAILS_ENV=${RAILS_ENV}
ENV RAILS_SERVE_STATIC_FILES true
ENV BUNDLE_PATH vendor/bundle
ENV BUNDLE_WITHOUT development:test
ENV RAILS_MASTER_KEY=${RAILS_MASTER_KEY}

RUN --mount=type=cache,id=apt-cache,sharing=locked,target=/var/cache/apt \
    --mount=type=cache,id=apt-lib,sharing=locked,target=/var/lib/apt \
    apt-get update -qq && \
    apt-get install --no-install-recommends -y \
    postgresql-client file vim curl gzip && \
    rm -rf /var/lib/apt/lists /var/cache/apt/archives

# Since I can't seem to specify the working directory for release command
# I am putting it in the root of the docker image.

# COPY --from=patio11-build-rails-app /app /app
COPY --from=tailscale /tailscale/tailscaled /usr/local/bin/tailscaled
COPY --from=tailscale /tailscale/tailscale /usr/local/bin/tailscale

WORKDIR /app

RUN mkdir -p tmp/pids

EXPOSE 8080

CMD ["/bin/fly_start.sh"]

fly_start.sh

#!/bin/sh

# Starts tailscale. Some uncertainty on whether this will work with multiple boxes.
tailscaled --state=/var/lib/tailscale/tailscaled.state --socket=/var/run/tailscale/tailscaled.sock &
tailscale up --authkey=${TAILSCALE_AUTHKEY} --hostname=fly-${FLY_APP_NAME}

# And now run the rails app.
bundle exec puma -C config/puma.rb