gentoo-layman/Dockerfile.template: Work around the lack of Python 2

gentoo-layman/Dockerfile.template: Work around the lack of Python 2

The existing stable version of Layman needs Python 2, but the ~amd64
version is compatible with Python 3:

  $ cd /usr/portage/app-portage/layman
  $ grep 'PYTHON_COMPAT\|KEYWORDS' *ebuild
  layman-2.0.0-r1.ebuild:PYTHON_COMPAT=( python{2_6,2_7} pypy2_0 )
  layman-2.0.0-r1.ebuild:KEYWORDS="... amd64 ..."
  layman-2.0.0-r3.ebuild:PYTHON_COMPAT=( python{2_6,2_7} pypy pypy2_0 )
  layman-2.0.0-r3.ebuild:KEYWORDS="... amd64 ..."
  layman-2.1.0-r2.ebuild:PYTHON_COMPAT=( python{2_7,3_3,3_4} pypy )
  layman-2.1.0-r2.ebuild:KEYWORDS="... ~amd64 ..."
  layman-9999.ebuild:PYTHON_COMPAT=( python{2_7,3_3,3_4} pypy )
  layman-9999.ebuild:KEYWORDS=""

Layman also needs Git, and with the 'python' USE flag enabled, Git
needs Python 2:

  $ cd /usr/portage/dev-vcs/git
  $ grep PYTHON_COMPAT *ebuild
  git-1.8.3.2-r1.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-1.8.4.5.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-1.8.5.5.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-1.9.3.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-2.0.1.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-2.0.2.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-2.0.4.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-9999-r1.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-9999-r2.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-9999-r3.ebuild:PYTHON_COMPAT=( python2_{6,7} )
  git-9999.ebuild:PYTHON_COMPAT=( python2_{6,7} )

The only interesting Git features enabled by the 'python' USE flag are
the Bazaar and Mercurial remote helpers, and even they were removed in
Git 2.0.0 [1,2].

[1]: https://git.kernel.org/cgit/git/git.git/commit/?id=b2c851a8e67da752d8a5dbde5a9dae6e3428a4c9
[2]: https://git.kernel.org/cgit/git/git.git/commit/?id=896ba14d653c43bd7a43b6b291e941ab60855c04