A Dockerfile for building a pants development image. (pantsbuild#4260)

Useful for chasing down linux-only problems on MacOS.

Note: Required a fix in bootstrap.sh. mktemp requires six X's
      at the end of a pattern (at least on Alpine Linux).

build-support/bin/native/bootstrap.sh

@@ -57,7 +57,7 @@ function ensure_build_prerequisites() {
   then
     log "A pants owned rustup installation could not be found, installing via the instructions at" \
         "https://www.rustup.rs ..."
-    local readonly rustup=$(mktemp -t pants.rustup.XXXXX)
+    local readonly rustup=$(mktemp -t pants.rustup.XXXXXX)
     curl https://sh.rustup.rs -sSf > ${rustup}
     sh ${rustup} -y --no-modify-path 1>&2
     rm -f ${rustup}

build-support/docker/pants_dev/Dockerfile

@@ -0,0 +1,37 @@
+# Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+# When developing Pants itself, errors sometimes manifest only on Linux.
+# However many developers work on MacOS and cannot reproduce such problems.
+# Docker provides a convenient way to access a Linux shell.
+
+# This Dockerfile builds a lightweight image that contains a bootstrapped
+# Pants repo, suitable for running Pants's own tests in.
+
+# To build this image:
+#   docker build -t pants_dev build-support/docker
+#   TBD: Push to a pants account on dockerhub.
+
+# To use on MacOS:
+#   - Install Docker for Mac (https://docs.docker.com/docker-for-mac/).
+#   - docker run -ti pants_dev
+
+# TODO: In the future this (or something like it) could be used to speed
+# up CIs, which currently have to do a lot of bootstrapping every time.
+
+# Note that we use Alpine Linux rather than, say, Ubuntu, because of its tiny size
+# and very snappy package installation.  We may need to re-think this, if it turns
+# out that running Pants on Alpine isn't adequately representative of the generic
+# Pants-on-Linux experience.
+
+# Installing JDK8 on Alpine Linux is a little convoluted,
+# so we start from an image that already includes it.
+FROM anapsix/alpine-java:8_jdk
+
+# Install various things Pants requires.
+RUN apk update && apk add bash curl gcc git libffi-dev linux-headers musl-dev openssl python python-dev
+
+# Clone the pantsbuild repo and bootstrap Pants in it.
+RUN git clone https://github.com/pantsbuild/pants.git && cd /pants && ./pants
+
+CMD bash