package-cache/Dockerfile.template: Use package.accept_keywords/package-cache

Avoid:

  Step 3 : RUN echo '=net-proxy/package-cache-9999 ~amd64' >> /etc/portage/package.accept_keywords
   ---> Running in f36c7b3c1a27
  /bin/sh: /etc/portage/package.accept_keywords: Is a directory
  2014/10/29 11:23:21 The command [/bin/sh -c echo '=net-proxy/package-cache-9999 ~amd64' >> /etc/portage/package.accept_keywords] returned a non-zero code: 1
  failed to build package-cache

Since d8ed6289 (gentoo-layman/Dockerfile.template: Work around the
lack of Python 2, 2014-08-30), gentoo-layman is creating directories
for /etc/portage/package.accept_keywords and /etc/portage/package.use.