Minimal Dockerfile for Alpine Linux, configurable Ruby version, no build tools, final image size about 200 M.

Dockerfile

FROM alpine:3.3
MAINTAINER Christopher Sexton <chris@radiusnetworks.com>

ENV RUBY_VERSION 2.3.0
ENV RUBY_DOWNLOAD_SHA1 96e620e38af351c8da63e40cfe217ec79f912ba1

ENV BASE_PACKAGES bash curl gmp
ENV BUILD_PACKAGES build-base libc-dev linux-headers openssl-dev postgresql-dev libxml2-dev libxslt-dev

# Update and install all of the required packages.
# At the end, remove the apk cache
RUN apk update && \
    apk upgrade && \
    apk add $BASE_PACKAGES && \
    rm -rf "/var/cache/apk/*"

RUN apk --update add --virtual build_deps $BUILD_PACKAGES && \
    cd /tmp && \
    wget -O ruby.tar.xz "https://cache.ruby-lang.org/pub/ruby/${RUBY_VERSION%.*}/ruby-$RUBY_VERSION.tar.xz" && \
    echo "$RUBY_DOWNLOAD_SHA1  ruby.tar.xz" | sha1sum -c - && \
    tar -xJf ruby.tar.xz && \
    rm ruby.tar.xz && \
    cd "/tmp/ruby-$RUBY_VERSION" && \
    ./configure --disable-install-doc && \
    make && \
    make install && \
    rm -rf "/tmp/ruby-$RUBY_VERSION" && \
    apk del build_deps && \
    rm -rf "/var/cache/apk/*"

RUN gem install bundler --no-rdoc --no-ri

RUN mkdir /app
WORKDIR /app
COPY Gemfile /app/
COPY Gemfile.lock /app/
RUN bundle install

COPY . /app