Dockerfile to build Ruby trunk

Dockerfile

# see https://github.com/docker-library/ruby/blob/e5d5ed23036c66c5797c39e0e156de5d006dbfc0/2.5/alpine3.7/Dockerfile
FROM alpine:3.7

# skip installing gem documentation
RUN mkdir -p /usr/local/etc \
	&& { \
		echo 'install: --no-document'; \
		echo 'update: --no-document'; \
	} >> /usr/local/etc/gemrc

ENV RUBYGEMS_VERSION 2.7.4
ENV BUNDLER_VERSION 1.16.1

# some of ruby's build scripts are written in ruby
#   we purge system ruby later to make sure our final image uses what we just built
# readline-dev vs libedit-dev: https://bugs.ruby-lang.org/issues/11869 and https://github.com/docker-library/ruby/issues/75
RUN set -ex \
	\
	&& apk add --no-cache --virtual .ruby-builddeps \
		autoconf \
		bison \
		bzip2 \
		bzip2-dev \
		ca-certificates \
		coreutils \
		dpkg-dev dpkg \
		gcc \
		gdbm-dev \
		glib-dev \
		libc-dev \
		libffi-dev \
		libressl \
		libressl-dev \
		libxml2-dev \
		libxslt-dev \
		linux-headers \
		make \
		ncurses-dev \
		procps \
		readline-dev \
		ruby \
		tar \
		xz \
		yaml-dev \
		zlib-dev \
                git \
	\
	&& git clone https://github.com/ruby/ruby.git \
	&& cd ruby \
	\
# hack in "ENABLE_PATH_CHECK" disabling to suppress:
#   warning: Insecure world writable dir
	&& { \
		echo '#define ENABLE_PATH_CHECK 0'; \
		echo; \
		cat file.c; \
	} > file.c.new \
	&& mv file.c.new file.c \
	\
	&& autoconf \
	&& gnuArch="$(dpkg-architecture --query DEB_BUILD_GNU_TYPE)" \
# the configure script does not detect isnan/isinf as macros
	&& export ac_cv_func_isnan=yes ac_cv_func_isinf=yes \
	&& ./configure \
		--build="$gnuArch" \
		--disable-install-doc \
		--enable-shared \
	&& make -j "$(nproc)" \
	&& make install \
	\
	&& runDeps="$( \
		scanelf --needed --nobanner --format '%n#p' --recursive /usr/local \
			| tr ',' '\n' \
			| sort -u \
			| awk 'system("[ -e /usr/local/lib/" $1 " ]") == 0 { next } { print "so:" $1 }' \
	)" \
	&& apk add --virtual .ruby-rundeps $runDeps \
		bzip2 \
		ca-certificates \
		libffi-dev \
		libressl-dev \
		procps \
		yaml-dev \
		zlib-dev \
	&& apk del .ruby-builddeps \
	&& cd / \
	&& gem update --system "$RUBYGEMS_VERSION" \
	&& gem install bundler --version "$BUNDLER_VERSION" --force

# install things globally, for great justice
# and don't create ".bundle" in all our apps
ENV GEM_HOME /usr/local/bundle
ENV BUNDLE_PATH="$GEM_HOME" \
	BUNDLE_BIN="$GEM_HOME/bin" \
	BUNDLE_SILENCE_ROOT_WARNING=1 \
	BUNDLE_APP_CONFIG="$GEM_HOME"
ENV PATH $BUNDLE_BIN:$PATH
RUN mkdir -p "$GEM_HOME" "$BUNDLE_BIN" \
	&& chmod 777 "$GEM_HOME" "$BUNDLE_BIN"