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Detection

migetpacks detects Ruby when any of these files are present in your project root:
  • Gemfile
  • Rakefile
  • config.ru

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Version Detection

Ruby version is resolved in this order:
PrioritySourceExample
1RUBY_VERSION env varRUBY_VERSION=3.3
2.ruby-version file3.4.4
3ruby declaration in Gemfileruby "3.3.0"
4Default3.4.4
The ruby- prefix is automatically stripped (e.g., ruby-3.2.0 becomes 3.2.0).

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Build Process

migetpacks generates a multi-stage Dockerfile with bundler layer caching: 
# Builder stage
FROM ruby:3.4 AS builder
WORKDIR /build

# Layer 1: Copy Gemfile for layer caching
COPY Gemfile Gemfile.lock ./

# Layer 2: Configure and install gems (CACHED when Gemfile unchanged)
ENV GEM_HOME=/build/vendor/bundle
ENV BUNDLE_PATH=/build/vendor/bundle
RUN gem install bundler -v '2.5.0' --no-document && \
    bundle config set --local path '/build/vendor/bundle' && \
    bundle config set --local deployment 'true' && \
    bundle install --jobs 4 --retry 3

# Layer 3: Copy source code
COPY . .

# Layer 4: Rails asset precompilation and cleanup
ENV RAILS_ENV=production
ENV SECRET_KEY_BASE_DUMMY=1
RUN bundle exec rake assets:precompile && \
    bundle exec rake assets:clean && \
    rm -rf .git test spec vendor/bundle/ruby/*/cache

# Runtime stage
FROM ruby:3.4-slim
RUN apt-get update && apt-get install -y libpq5 tzdata ca-certificates
WORKDIR /app
COPY --from=builder /build /app

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Dependency Detection

migetpacks inspects your Gemfile and Gemfile.lock to install the correct system libraries:
GemBuild PackageRuntime Package
pglibpq-devlibpq5
mysql2, trilogylibmariadb-devlibmariadb3
sqlite3libsqlite3-devlibsqlite3-0
nokogiri, railslibxml2-dev, libxslt1-devlibxml2, libxslt1.1
Additionally, tzdata and ca-certificates are always installed in the runtime image.

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Bundler Version

The bundler version is automatically detected from Gemfile.lock (the BUNDLED WITH section) and installed in the builder stage.

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Local Gems

If your Gemfile references local gems with path: options, those directories are automatically copied before bundle install to ensure they are available during gem resolution.

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Rails Asset Precompilation

If your project has a Rakefile and either app/assets/ or app/javascript/ directories, migetpacks automatically:
  1. Sets RAILS_ENV=production, SECRET_KEY_BASE_DUMMY=1, and a dummy DATABASE_URL
  2. Runs bundle exec rake assets:precompile
  3. Runs bundle exec rake assets:clean

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Runtime Cleanup

The following are removed from the final image:
  • .git/, .github/
  • test/, tests/, spec/, features/
  • .rspec, .rubocop*
  • vendor/bundle/ruby/*/cache/

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Run Command

The default run command is determined in this order:
PrioritySourceCommand
1RUN_COMMAND env varUser-specified
2web: in ProcfileFrom Procfile
3config.ru existsbundle exec puma -C config/puma.rb
4Defaultbundle exec ruby app.rb

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Runtime Environment

The following environment variables are set in the runtime container: 
RAILS_ENV=production
RACK_ENV=production
RAILS_SERVE_STATIC_FILES=enabled
RAILS_LOG_TO_STDOUT=enabled
MALLOC_ARENA_MAX=2
DISABLE_SPRING=1
PUMA_PERSISTENT_TIMEOUT=95

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Caching

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Docker Layer Caching

Gems are installed in a separate layer before source code is copied. bundle install is only re-run when Gemfile or Gemfile.lock changes.

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BuildKit Cache Mounts

When BUILD_CACHE_DIR is configured, BuildKit cache mounts are used:
Cache PathContents
/cache/bundlerBundler gem cache

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Registry Cache

Use CACHE_IMAGE to push/pull BuildKit inline cache layers to a registry for cross-build caching.

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DHI Support

Ruby is fully supported with Docker Hardened Images.
StageImage
Builddhi.io/ruby:{version}-dev
Runtimedhi.io/ruby:{version}

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DHI-Specific Behavior

The DHI runtime is distroless (no shell, no apt-get), which requires special handling: Native gem libraries are copied from the builder stage: 
COPY --from=builder /usr/lib/*/libpq.so* /usr/lib/
COPY --from=builder /usr/share/zoneinfo /usr/share/zoneinfo
COPY --from=builder /etc/ssl/certs /etc/ssl/certs
Process commands are transformed since shebangs do not work in distroless:
  • ./bin/rails server becomes ruby bin/rails server
  • bundle exec commands work because bundler is installed as a gem
BUNDLER_VERSION is explicitly set in the environment to prevent bundler from attempting auto-switching (which fails without /usr/bin/env).
Shell-dependent features like .profile.d scripts and shell variable expansion (${PORT:-5000}) are not available in DHI runtime images. Use environment variables directly.

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Multi-Buildpack

Ruby applications commonly use Node.js for asset compilation. When package.json is detected alongside a Ruby project, Node.js is automatically added as a secondary buildpack: 
# Explicit buildpack ordering
docker run --rm \
  -v /path/to/app:/workspace/source \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -e OUTPUT_IMAGE=my-rails-app:latest \
  -e BUILDPACKS=ruby,nodejs \
  miget/migetpacks:latest
See the Multi-Buildpack guide for more details.

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Example

docker run --rm \
  -v /path/to/app:/workspace/source \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -e OUTPUT_IMAGE=my-ruby-app:latest \
  miget/migetpacks:latest

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With Custom Options

docker run --rm \
  -v /path/to/app:/workspace/source \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -e OUTPUT_IMAGE=registry.io/app:v1 \
  -e RUBY_VERSION=3.3 \
  -e RAILS_MASTER_KEY=abc123 \
  -e BUNDLE_WITHOUT="development test" \
  -e USE_DHI=true \
  miget/migetpacks:latest