With the https://github.com/pantsbuild/example-python main checkout,

diff --git a/BUILD b/BUILD
index fca77e4..0acfb4b 100644
--- a/BUILD
+++ b/BUILD
@@ -5,3 +5,10 @@
 # `python_requirement_library` target. Refer to
 # <https://www.pantsbuild.org/docs/python-third-party-dependencies>.
 python_requirements(name="reqs")
+
+docker_environment(
+  name="ubuntu-base",
+  platform="linux_x86_64",
+  image="ubuntu",
+  python_bootstrap_search_path=["<PATH>"],
+)
diff --git a/helloworld/BUILD b/helloworld/BUILD
index 5e55812..c882cf7 100644
--- a/helloworld/BUILD
+++ b/helloworld/BUILD
@@ -13,4 +13,5 @@ python_sources(
 pex_binary(
     name="pex_binary",
     entry_point="main.py",
+    environment="linux_docker"
 )
diff --git a/pants.toml b/pants.toml
index d0981a6..f15b626 100644
--- a/pants.toml
+++ b/pants.toml
@@ -44,3 +44,6 @@ resolves = { python-default = "python-default.lock"}
 #  problematic system Pythons. See
 #  <https://www.pantsbuild.org/docs/python-interpreter-compatibility#changing-the-interpreter-search-path>.
 search_path = ["<PATH>", "<PYENV>"]
+
+[environments-preview.names]
+linux_docker = "//:ubuntu-base"

I have

$ pants package ::
14:31:27.08 [INFO] Completed: Building local_dists.pex
14:31:27.08 [ERROR] 1 Exception encountered:

Engine traceback:
  in `package` goal

ProcessExecutionFailure: Process 'Building local_dists.pex' failed with exit code 1.
stdout:

stderr:
Failed to find a compatible PEX_PYTHON=.python-build-standalone/python/bin/python3.

No interpreters could be found on the system.

inside the sandbox I've asked to keep on failure, I see a symlink

.python-build-standalone -> /pants-immutable-inputs/.tmpctvJXn/c12164f0e9228ec20704c1aba97eb31b8e2a482d41943d541cc8e3a9e84f7349

Tried to set

export PEX_PYTHON=/usr/bin/python3;

before running Pants and pointing to them in

docker_environment(
  name="ubuntu-base",
  platform="linux_x86_64",
  image="ubuntu",
  python_bootstrap_search_path=["<PATH>"],
  docker_env_vars=["PEX_PYTHON"],
  subprocess_environment_env_vars=["PEX_PYTHON"],
)

but same result.