create image to build images with pack CLI

- copied this from the BOSH team:
  - https://github.com/cloudfoundry/bosh/blob/master/ci/old-docker/main-bosh-docker
- ideally should be able to remove everything besides downloading the
`pack` CLI after the following issue in the `pack` repo is resolved:
  - buildpacks/pack#564

[#172847711]

capi-pack-builder/Dockerfile

@@ -0,0 +1,26 @@
+FROM ubuntu:xenial
+
+RUN apt-get update
+RUN apt-get install -y --no-install-recommends \
+  apt-transport-https \
+  ca-certificates \
+  curl \
+  dnsutils \
+  git \
+  iproute2 \
+  openssh-client \
+  software-properties-common \
+  libpq-dev
+
+RUN curl -fsSL https://download.docker.com/linux/ubuntu/gpg | apt-key add -
+RUN apt-key fingerprint | grep 'Key fingerprint = 9DC8 5822 9FC7 DD38 854A  E2D8 8D81 803C 0EBF CD88'
+RUN add-apt-repository "deb https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
+RUN apt-get update
+RUN apt-get install -y --no-install-recommends docker-ce
+
+RUN rm -rf /var/lib/apt/lists/*
+
+RUN curl -LO https://github.com/buildpacks/pack/releases/download/v0.10.0/pack-v0.10.0-linux.tgz
+RUN tar xvf pack-v0.10.0-linux.tgz
+RUN mv pack /usr/local/bin/pack
+RUN rm pack-v0.10.0-linux.tgz