The Skinny Ruby Queen: Minimal Dockerfile for Production

1. Build Stage

• We're building in style. Ruby + Alpine = skinny legend

FROM ruby:3.3-alpine AS build

• Install a full dev toolchain to compile native gems (yes, Ruby still lives in C land)

RUN apk add --no-cache build-base

• Set the working directory—aka the sacred ground where it all happens

WORKDIR /usr/src/app

• Copy only Gemfile and lockfile first (layer caching magic)

COPY Gemfile Gemfile.lock ./

• Configure bundler to install gems locally under vendor/bundle. This will be copied over to the final image later like a blessed artifact

RUN bundle config set --local path 'vendor/bundle' \
  && bundle install

• Copy the rest of your application—code, chaos, and all

COPY . .

2. Final Stage

• A clean Alpine base with Ruby and none of that build baggage. We like our containers light.

FROM ruby:3.3-alpine

• Set the working dir again (yes, you need to re-declare it—Docker has no memory of its past life)

WORKDIR /usr/src/app

• Copy everything from the build stage, including those precious compiled gems

COPY --from=build /usr/src/app /usr/src/app

• Let Ruby know where the gems are—because it forgets if you don’t tell it

ENV GEM_PATH=/usr/src/app/vendor/bundle/ruby/3.3.0
ENV PATH=$GEM_PATH/bin:$PATH

• Install only the runtime dependencies needed for your app to vibe

RUN apk add --no-cache \
        libstdc++ \       # C++ runtime
        libffi \          # Needed by some gems (e.g., FFI, psych)
        yaml \            # YAML parsing
        zlib \            # Compression stuff
        openssl \         # HTTPS, TLS, etc.
        tzdata            # So your logs don’t think it's 1970

• Declare yourself: prod mode on

ENV RACK_ENV=production
ENV PORT=8080
EXPOSE 8080

• Finally, launch the Ruby app like the main character it is

CMD ["ruby", "server.rb"]

Some useful commands:

docker build -t your-image-name .

docker images

docker run -p 8080:8080 your-image-id

docker rmi your-image-id

docker container prune

Pro Tips from the Underworld: If you're using gems that compile C extensions (like pg, nokogiri, ffi), you’ll likely need additional Alpine dependencies, e.g.:

RUN apk add --no-cache build-base libxml2-dev libxslt-dev postgresql-dev

For scripts that are long-running, consider using:

CMD ["ruby", "start.rb"]

Or even:

CMD ["rackup", "--host", "0.0.0.0", "--port", "8080"]