chore: refactor Dockerfile

- change base image to ubuntu:20.04
- support ruby 2.7
- docker lint

Dockerfile

@@ -1,3 +1,5 @@
+# syntax=docker/dockerfile:1
+
 # Are we running against the minimal container, or the everything
 # container? Minimal is mostly the compiled package tools. Everything
 # pulls in scripting langauges.
@@ -8,87 +10,98 @@ ARG BASE_ENV=everything
 ARG TARGET=test
 
 # Container to throw an error if called with a bare `docker build .`
-FROM ubuntu:18.04 as error
-RUN echo "\n\n\nHey! Use buildkit. See the Makefile or docs\n\n\n"
-RUN false
+FROM ubuntu:20.04 as error
+RUN <<EOF
+  printf '\n\n\n%s\n\n\n' "Hey! Use buildkit. See the Makefile or docs"
+  false
+EOF
 
 # Base container is used for various release and test things
-FROM ubuntu:18.04 as minimal-base
-
+FROM ubuntu:20.04 as minimal-base
+ARG DEBIAN_FRONTEND=noninteractive
+ARG TZ=Etc/UTC
 # Runtime deps. Build deps go in the build or test containers
-RUN apt-get update \
-	&& apt-get -y dist-upgrade \
-	&& apt-get install --no-install-recommends -y \
-	ruby rubygems rubygems-integration \
-	bsdtar \
-	cpio \
-	debsigs \
-	pacman \
-	rpm  \
-	squashfs-tools \
-	xz-utils \
-	zip \
-	&& rm -rf /var/lib/apt/lists/* \
-	&& apt-get clean
-RUN adduser fpm 
+# hadolint ignore=DL3009
+RUN <<EOF
+  apt-get update
+  apt-get install --no-install-recommends --no-install-suggests -y \
+    'ruby=*' \
+    'ruby-dev=*' \
+    'libarchive-tools=*' \
+    'cpio=*' \
+    'debsigs=*' \
+    'pacman=*' \
+    'rpm=*'  \
+    'squashfs-tools=*' \
+    'xz-utils=*' \
+    'zip=*' \
+    'gcc=*' \
+    'libc6-dev=*' \
+    'make=*' \
+    'lintian=*' \
+    'git=*'
+    useradd -ms /bin/bash fpm
+EOF
 
 # everything container includes all the scripting languages. These
 # greatly embiggen the underlying docker container, so they're
 # conditionalized.
-FROM minimal-base AS everything-base
-RUN apt-get update \
-	&& apt-get install --no-install-recommends -y \
-	cpanminus \
-	npm \
-	perl \
-	python3-pip \
-	&& pip3 --no-cache-dir install setuptools \
-	&& pip3 --no-cache-dir install wheel \
-	&& pip3 --no-cache-dir install virtualenv virtualenv-tools3 \
-	&& update-alternatives --install /usr/bin/python python /usr/bin/python3 10 \
-	&& rm -rf /var/lib/apt/lists/*
+FROM minimal-base as everything-base
+RUN <<EOF
+  apt-get install --no-install-recommends --no-install-suggests -y \
+    'cpanminus=*' \
+    'npm=*' \
+    'perl=*' \
+    'python3-pip=*'
+  pip3 --no-cache-dir install 'setuptools>=45' 'wheel>=0.34' 'virtualenv>=20' 'virtualenv-tools3>=2'
+  update-alternatives --install /usr/bin/python python /usr/bin/python3 10
+EOF
 
+# hadolint ignore=DL3006
+FROM ${BASE_ENV}-base as base
+RUN <<EOF
+  rm -rf /var/lib/apt/lists/*
+  apt-get clean
+EOF
 
 # Run tests against the current working directory. This is a bit
 # orthogonal to the container release process, but it has a lot of
 # same dependancies, so we reuse it. This uses COPY to allow rspect to
 # initall the gems, but runtime usage expects you to mount a volume
 # into /src
-FROM ${BASE_ENV}-base AS test
-WORKDIR /src
-RUN apt-get update \
-	&& apt-get install --no-install-recommends -y \
-	gcc make ruby-dev libc-dev lintian git
+FROM base AS test
 # installing ffi here is a bit of an optimization for how COPY and layer reuse works
-RUN gem install --no-ri --no-rdoc ffi
-
-RUN install -d -o fpm /origsrc
-COPY --chown=fpm . /origsrc
+RUN gem install --no-document ffi:*
+USER fpm
+WORKDIR /origsrc
 ENV HOME=/origsrc
 ENV BUNDLE_PATH=/origsrc/.bundle
-# Install a specific version of bundler
-WORKDIR /origsrc
-RUN gem install -v "$(grep -A1 '^BUNDLED WITH' Gemfile.lock | tail -1)" bundler
-USER fpm
-RUN bundle install
-CMD bundle exec rspec
+SHELL ["/bin/bash", "-o", "pipefail", "-c"]
+RUN <<EOF
+  # Install a specific version of bundler
+  install -d -o fpm /origsrc
+  gem install -v "$(grep -A1 '^BUNDLED WITH' Gemfile.lock | tail -1)" bundler:*
+  bundle install
+EOF
+
+CMD ["bundle", "exec", "rspec"]
 
 # build a container from a released gem. install build deps here, so
 # we can omit them from the final release package
-FROM ${BASE_ENV}-base AS build
-RUN apt-get update
-RUN apt-get install --no-install-recommends -y \
-	gcc make ruby-dev libc-dev
-ENV GEM_PATH /fpm
-ENV PATH "/fpm/bin:${PATH}"
-RUN gem install --no-ri --no-rdoc --install-dir=/fpm fpm
+FROM base AS build
+ENV GEM_PATH=/fpm
+ENV PATH="/fpm/bin:${PATH}"
+# hadolint ignore=DL3028
+RUN gem install --no-document --install-dir=/fpm fpm
 
-FROM build as release
+FROM base as release
 COPY --from=build /fpm /fpm
-ENV GEM_PATH /fpm
-ENV PATH "/fpm/bin:${PATH}"
+ENV GEM_PATH=/fpm
+ENV PATH="/fpm/bin:${PATH}"
+USER fpm
 WORKDIR /src
 ENTRYPOINT ["/fpm/bin/fpm"]
 
 # This target is to help docker buildkit in resolving things.
-FROM ${TARGET} as final
+# hadolint ignore=DL3006
+FROM ${TARGET}