Move interpreter constraints to Docker env entry

Docker does not pull in external env vars. Instead, we must specify this in the Dockerfile.

This change has added benefit that it moves all of the Py2 logic into the Dockerfile out of .travis.yml, and leaves .travis.yml solely to call the Dockerfile.

.travis.yml

@@ -400,9 +400,6 @@ py27_linux_build_wheels_ucs2: &py27_linux_build_wheels_ucs2
   name: "Build wheels - Linux and cp27m (UCS2)"
   env:
     - *base_build_wheels_env
-    # While we install 2.7.15 with UCS2, the Centos6 image already has 2.7.13 (UCS4). To disambiguate
-    # what `python2` resolves to and ensure we always use UCS2, we require exactly 2.7.15.
-    - PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS="['CPython==2.7.15']"
     - CACHE_NAME=linuxwheelsbuild.ucs2
   script:
     - docker build --rm -t travis_ci_py27_ucs2

build-support/docker/travis_ci_py27_ucs2/Dockerfile

@@ -9,7 +9,7 @@ FROM pantsbuild/centos6:latest
 
 # Note we use 2.7.15, rather than 2.7.13, as the centos6 image already comes with 2.7.13
 # installed, which uses UCS4 instead of UCS2. We pair this change with setting
-# PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS to exactly 2.7.15 in `.travis.yml` to disambiguate
+# PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS to exactly 2.7.15 to disambiguate
 # the two and always choose the UCS2 interpeter.
 ARG PYTHON_2_VERSION=2.7.15
 RUN yum install sqlite-devel -y
@@ -21,6 +21,7 @@ RUN /usr/bin/scl enable devtoolset-7 -- bash -c '\
     ${PYENV_ROOT}/bin/pyenv install ${PYTHON_2_VERSION} \
     && ${PYENV_ROOT}/bin/pyenv global ${PYTHON_2_VERSION}'
 ENV PATH "${PYENV_ROOT}/shims:${PATH}"
+ENV PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS "['CPython==${PYTHON_2_VERSION}']"
 
 # Setup mount points for the travis ci user & workdir.
 VOLUME /travis/home

build-support/travis/travis.yml.mustache

@@ -379,9 +379,6 @@ py27_linux_build_wheels_ucs2: &py27_linux_build_wheels_ucs2
   name: "Build wheels - Linux and cp27m (UCS2)"
   env:
     - *base_build_wheels_env
-    # While we install 2.7.15 with UCS2, the Centos6 image already has 2.7.13 (UCS4). To disambiguate
-    # what `python2` resolves to and ensure we always use UCS2, we require exactly 2.7.15.
-    - PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS="['CPython==2.7.15']"
     - CACHE_NAME=linuxwheelsbuild.ucs2
   script:
     - docker build --rm -t travis_ci_py27_ucs2