Ah, nm. I'm always too slow to pull out the right tool for this sort of debugging. With this Dockerfile:

FROM ubuntu:18.04

RUN apt update && apt upgrade -y && apt install -y locales language-pack-en
ENV LANG=en_US.UTF-8 LANGUAGE=en_US:en LC_ALL=en_US.UTF-8

RUN apt install -y curl python python3-dev git vim build-essential unzip tar

I reproduce using:

$ docker run --rm -it raul:3.6
root@f1a736edc580:/# git clone <https://github.com/pantsbuild/example-python>
Cloning into 'example-python'...
remote: Enumerating objects: 329, done.
remote: Total 329 (delta 0), reused 0 (delta 0), pack-reused 329
Receiving objects: 100% (329/329), 74.15 KiB | 1.65 MiB/s, done.
Resolving deltas: 100% (197/197), done.
root@f1a736edc580:/# cd example-python
root@f1a736edc580:/example-python# ./pants fmt lint typecheck test ::
...
New virtual environment successfully created at /root/.cache/pants/setup/bootstrap-Linux-x86_64/2.0.0_py36.
17:11:46.96 [INFO] initializing pantsd...
17:11:47.83 [INFO] pantsd initialized.
17:11:48.02 [INFO] No pyenv binary found. Will not use pyenv interpreters.
17:11:54.18 [INFO] Completed: Building docformatter.pex with 1 requirement: docformatter>=1.3.1,<1.4
17:11:54.53 [INFO] Completed: Format with docformatter - made no changes.
17:11:54.88 [ERROR] Exception caught: (pants.engine.internals.scheduler.ExecutionError)
  File "/root/.cache/pants/setup/bootstrap-Linux-x86_64/2.0.0_py36/lib/python3.6/site-packages/pants/bin/local_pants_runner.py", line 289, in run
    engine_result = self._run_v2()
  File "/root/.cache/pants/setup/bootstrap-Linux-x86_64/2.0.0_py36/lib/python3.6/site-packages/pants/bin/local_pants_runner.py", line 195, in _run_v2
    return self._maybe_run_v2_body(goals, poll=False)
  File "/root/.cache/pants/setup/bootstrap-Linux-x86_64/2.0.0_py36/lib/python3.6/site-packages/pants/bin/local_pants_runner.py", line 217, in _maybe_run_v2_body
    poll_delay=(0.1 if poll else None),
  File "/root/.cache/pants/setup/bootstrap-Linux-x86_64/2.0.0_py36/lib/python3.6/site-packages/pants/init/engine_initializer.py", line 127, in run_goal_rules
    goal_product, params, poll=poll, poll_delay=poll_delay
  File "/root/.cache/pants/setup/bootstrap-Linux-x86_64/2.0.0_py36/lib/python3.6/site-packages/pants/engine/internals/scheduler.py", line 569, in run_goal_rule
    self._raise_on_error([t for _, t in throws])
  File "/root/.cache/pants/setup/bootstrap-Linux-x86_64/2.0.0_py36/lib/python3.6/site-packages/pants/engine/internals/scheduler.py", line 539, in _raise_on_error
    wrapped_exceptions=tuple(t.exc for t in throws),

Exception message: 1 Exception encountered:

  ProcessExecutionFailure: Process 'Find interpreter for constraints: CPython>=3.6' failed with exit code 102.
stdout:

stderr:
Could not find a compatible interpreter.

Examined the following interpreters:
1.) /usr/bin/python2.7 CPython==2.7.17

No interpreter compatible with the requested constraints was found:
  Version matches CPython>=3.6



(Use --print-stacktrace to see more error details.)
root@f1a736edc580:/example-python#