Add a 4.2.1 dockerfile based on @futurejones'

Use the @futurejones package for ARM 64bit image of Swift 4.2.1.

arm64v8-4.2.1-futurejones/Makefile

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+# Makefile
+
+all:
+	@echo "use 'build' target"
+
+build:
+	time docker build -t helje5/arm64v8-swift:4.2.1 \
+	     -f arm64v8-ubuntu-swift-4.2.1.dockerfile \
+	     $(PWD)/../empty-ctx
+	docker images | grep helje5/arm64v8
+
+publish:
+	time docker push helje5/arm64v8-swift:4.2.1

arm64v8-4.2.1-futurejones/README.md

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+# Personal Notes on Building the image
+
+
+Packages:
+
+- https://packagecloud.io/swift-arm/release/packages/ubuntu/bionic/swift4_4.2.1_arm64.deb

arm64v8-4.2.1-futurejones/arm64v8-ubuntu-swift-4.2.1.dockerfile

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+# Dockerfile
+#
+#   docker build -t helje5/arm64v8-swift:4.2.1 \
+#     -f arm64v8-4.2.0-futurejones/arm64v8-ubuntu-swift-4.2.1.dockerfile \
+#     ./empty-ctx
+#   docker run -i --tty --rm helje5/arm64v8-swift:4.2.1
+# 
+FROM arm64v8/ubuntu:18.04
+
+LABEL maintainer "Helge Heß <me@helgehess.eu>"
+
+ARG SWIFTPKG=https://packagecloud.io/swift-arm/release/packages/ubuntu/bionic/swift4_4.2.1_arm64.deb
+
+ENV DEBIAN_FRONTEND noninteractive
+
+RUN apt-get update
+RUN apt-get upgrade -y
+
+RUN apt-get install -y \
+  git           \
+  libedit2      \
+  libpython2.7 libcurl4 libxml2 libicu60 \
+  libc6-dev     \
+  libatomic1    \
+  libpython3.5  \
+  curl
+
+RUN bash -c "curl -s https://packagecloud.io/install/repositories/swift-arm/release/script.deb.sh | bash"
+
+RUN apt-get install -y swift4=4.2.1
+
+RUN bash -c "echo '/usr/lib/swift/linux' > /etc/ld.so.conf.d/swift.conf;\
+             echo '/usr/lib/swift/clang/lib/linux' >> /etc/ld.so.conf.d/swift.conf;\
+             echo '/usr/lib/swift/pm' >> /etc/ld.so.conf.d/swift.conf;\
+             ldconfig"
+
+RUN useradd -u 501 --create-home --shell /bin/bash swift
+
+USER swift
+WORKDIR /home/swift