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Container Builds

Optimal Dockerfile for Ruby on Rails with Bundler

Below is an example Dockerfile that we recommend at Depot for building images for Ruby on Rails applications with Bundler.

# syntax=docker/dockerfile:1

FROM ruby:3.4 AS build

WORKDIR /app

ENV RAILS_ENV=production

COPY Gemfile ./

RUN bundle config set --local without 'development test' && \
    bundle config set --local jobs $(nproc)

RUN --mount=type=cache,target=/usr/local/bundle/cache \
    --mount=type=cache,target=/app/vendor/cache \
    bundle cache && \
    bundle install && \
    bundle clean --force

COPY . .

FROM ruby:3.4-slim AS runtime

RUN groupadd -g 1001 appgroup && \
    useradd -u 1001 -g appgroup -m -d /home/appuser -s /bin/bash appuser

WORKDIR /app

COPY --from=build --chown=appuser:appgroup /app .
COPY --from=build --chown=appuser:appgroup /usr/local/bundle /usr/local/bundle

RUN mkdir -p tmp/pids tmp/cache log storage && \
    chown -R appuser:appgroup tmp log storage

ENV RAILS_ENV=production \
    RUBY_YJIT_ENABLE=1 \
    BUNDLE_WITHOUT=development:test

USER appuser

ENTRYPOINT ["bundle", "exec", "puma", "-C", "config/puma.rb"]

Explanation of the Dockerfile

At a high level, here are the things we're optimizing in our Docker build for a Ruby application with Bundler:

  • Multi-stage builds for cleaner separation
  • Bundler cache mounts for faster dependency installation
  • YJIT enabled for improved Ruby performance
  • Security optimizations with non-root users

Stage 1: FROM ruby:3.4 AS build

FROM ruby:3.4 AS build

We use the official Ruby 3.4 image as our build stage base. This provides a full Ruby environment with all necessary build tools for compiling native gems.

Environment and dependency configuration

WORKDIR /app

ENV RAILS_ENV=production

COPY Gemfile ./

RUN bundle config set --local without 'development test' && \
    bundle config set --local jobs $(nproc)

We set the production environment and configure Bundler:

  • without 'development test' excludes development and test gems
  • jobs $(nproc) enables parallel gem installation using all available CPU cores

Gem installation with caching

RUN --mount=type=cache,target=/usr/local/bundle/cache \
    --mount=type=cache,target=/app/vendor/cache \
    bundle cache && \
    bundle install && \
    bundle clean --force

We install gems with dual cache mounts for maximum build efficiency:

  • bundle cache downloads and caches gems locally before installation
  • bundle install installs the cached gems
  • bundle clean --force removes any gems not in the current Gemfile, keeping the installation clean

The dual cache mounts optimize both Bundler's internal cache and the vendor cache directory.

COPY . .

Stage 2: FROM ruby:3.4-slim AS runtime

FROM ruby:3.4-slim AS runtime

RUN groupadd -g 1001 appgroup && \
    useradd -u 1001 -g appgroup -m -d /home/appuser -s /bin/bash appuser

The runtime stage uses Ruby 3.4 slim image for a smaller footprint and creates a non-root user for security:

  • groupadd creates a group with GID 1001
  • useradd creates a user with UID 1001, home directory, and bash shell
WORKDIR /app

COPY --from=build --chown=appuser:appgroup /app .
COPY --from=build --chown=appuser:appgroup /usr/local/bundle /usr/local/bundle

We copy the application and installed gems from the build stage with proper ownership.

Application setup and permissions

RUN mkdir -p tmp/pids tmp/cache log storage && \
    chown -R appuser:appgroup tmp log storage

ENV RAILS_ENV=production \
    RUBY_YJIT_ENABLE=1 \
    BUNDLE_WITHOUT=development:test

USER appuser

ENTRYPOINT ["bundle", "exec", "puma", "-C", "config/puma.rb"]

We create necessary directories for Rails runtime files (PIDs, cache, logs, storage) with correct permissions and configure the runtime environment:

  • RAILS_ENV=production sets the Rails environment
  • RUBY_YJIT_ENABLE=1 enables YJIT for improved Ruby performance (Ruby 3.1+)
  • BUNDLE_WITHOUT=development:test ensures development gems aren't loaded
  • Puma web server with configuration file

Understanding BuildKit Cache Mounts

Cache mounts are one of the most powerful features for optimizing Docker builds with Depot. This Dockerfile uses dual cache mounts:

RUN --mount=type=cache,target=/usr/local/bundle/cache \
    --mount=type=cache,target=/app/vendor/cache \
    bundle cache && \
    bundle install && \
    bundle clean --force

Cache Mount Parameters Explained

  • type=cache: Specifies this is a cache mount that persists across builds.

  • Bundler cache mounts:

    • target=/usr/local/bundle/cache: Mount point for Bundler's internal gem cache
    • target=/app/vendor/cache: Mount point for vendored gem cache

For more information regarding Bundler cache mounts, please visit the official Bundler documentation.