Make Pex and Docker integration easy.

[Nishikoh]

Is your feature request related to a problem? Please describe.

This idea is to make Docker and pex integration user-friendly: if we can get the pex target in an ARG and copy it, the user can integrate it while keeping the pex path hidden.

Describe the solution you'd like

Pex binary path is received as an argument at build time to simplify COPY. The following uses are envisioned.

# BUILD
pex_binary(name="hello", entry_point="hello_world.py")

# Docker
FROM python:3.12
ARG PEX_BIN=:hello
COPY ${PEX_BIN} /app/hello.pex

Currently, it is necessary to specify the pex path explicitly, as follows.
COPY foo.bar.baz/hello.pex /app/hello.pex

Additional context

As the base image target can already be specified, it may be possible to apply it to the pex target.

ARG BASE_IMAGE=:base
FROM $BASE_IMAGE

ref: https://www.pantsbuild.org/2.19/docs/docker#dependency-inference-support