Permission denied when running Ruby on Rails in a docker container - ruby-on-rails

Question

Trying to run a ruby on rails app, here is the docker-compose.yml:
version: '3'
services:
postgres:
image: postgres:11
volumes:
- ./tmp/db:/app/tmp/db
environment:
POSTGRES_HOST_AUTH_METHOD: trust
app:
build: .
command: bash -c "yarn install --check-files ; rm -f tmp/pids/server.pid ; /app/bin/webpack-dev-server --host 0.0.0.0 & bundle exec rails s -p 3000 -b '0.0.0.0'"
volumes:
- .:/app
- /home/$USER/.ssh:/root/.ssh
ports:
- "3000:3000"
- "3035:3035"
depends_on:
- postgres
- solr
solr:
image: solr:8.2
And here is the Dockerfile:
FROM <OMITTED>/ruby_2.5.1:latest
MAINTAINER <OMITTED>
# Install apt-get dependencies and nodejs
RUN apt-get update && apt-get install -y \
build-essential
# Set working directory for following commands
RUN mkdir -p /app
WORKDIR /app
# Copy Gemfile and then install gems through bundler
COPY Gemfile Gemfile.lock ./
RUN gem install bundler && bundle install
# Installing nodejs, yarn, and the java runtime environment
RUN curl -sL https://deb.nodesource.com/setup_10.x | bash -
RUN apt-get install -y nodejs
RUN curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add -
RUN echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list
RUN wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | apt-key add -
RUN echo "deb http://apt.postgresql.org/pub/repos/apt/ `lsb_release -cs`-pgdg main" | tee /etc/apt/sources.list.d/pgdg.list
RUN apt-get update && apt-get install -y yarn postgresql-client-12
RUN yarn install --check-files
RUN apt-get install -y default-jre ffmpeg nano
RUN apt-get install -y ruby-chromedriver-helper
# Copy the main application.
COPY . ./
# Start SSH agent and add key
RUN echo 'eval "$(ssh-agent -s)" &>/dev/null' >> ~/.bashrc
RUN echo 'ssh-add $(find /root/.ssh -type f ! -name "*.*" | grep id) &>/dev/null' >> ~/.bashrc
RUN yarn install --check-files
#Searches in the /.ssh directory in the container for files that do not have an extension,
#searches them for 'id' which will be a key, and then adds to the ssh agent
RUN ln -sf /usr/local/rvm/rubies/ruby-2.5.1/bin/* /usr/local/bin
# Expose port 3000 so it can be seen outside of the container
EXPOSE 3000
EXPOSE 3035
# Run the rails server
CMD ["rails", "server"]
The project is a pretty standard ruby on rails app; it uses postgresql for the database, solr for search functionality on the website, and yarn for javascript packages. When I run docker-compose up, the project builds successfully (gets through all 25 steps in the Dockerfile), the solr and postgresql images start up fine, but when it comes to the web app itself I get this error:
app_1 | yarn install v1.22.18
app_1 | Error: EACCES: permission denied, open '/app/package.json'
app_1 | at Object.openSync (fs.js:462:3)
app_1 | at Object.readFileSync (fs.js:364:35)
app_1 | at onUnexpectedError (/usr/share/yarn/lib/cli.js:88608:106)
app_1 | at /usr/share/yarn/lib/cli.js:88727:9
app_1 | bash: line 1: /app/bin/webpack-dev-server: Permission denied
app_1 | Your RubyGems version (3.0.9) has a bug that prevents `required_ruby_version` from working for Bundler. Any scripts that use `gem install bundler` will break as soon as Bundler drops support for your Ruby version. Please upgrade RubyGems to avoid future breakage and silence this warning by running `gem update --system 3.2.3`
app_1 | Could not locate Gemfile or .bundle/ directory
<OMITTED>-rails_app_1 exited with code 10
I've tried so far:
Setting permissions (sudo chown -R $USER:$USER . and variations)
Pruning docker (running docker system prune -a)
Changing git branches (I originally thought it was some code on my feature branch causing an issue)
Rebooting computer and updating packages
None of these have remedied the issue.