Add `entrypoint` routing, which supports routing string-based commands to different executables, with special logic for `pwsh`

Goal

Add entrypoint routing, which supports routing string-based commands to different executables, with special logic for pwsh. In order to:

• Provide actionable error messages. Users should know that the entrypoint syntax they used failed in PowerShell, in Bash (or other shell), or in the script itself.
• Support on Linux and Windows (a number of these appear like general concerns, not Linux-specific).
• This avoids needing to
  • use the --entrypoint syntax to specify an executable, and
  • split the entrypoint (an argument to docker run) from its arguments (arguments to the container; they show up last).

Background

The best practice in docker is for an image to run the product in contains.
https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#entrypoint

Currently our images only run PowerShell when no command is specified. This is specified in the CMD entry in the Dockerfile.

Unfortunately, making the change causes a breaking change to the image in what is hopefully a corner case.

Customer requests for this are in #274 and #394

Proof of concept is here: https://github.com/PowerShell/PowerShell-Docker/pull/399

Other Language images

• Work similar to the proposal
  • php
  • node
• Like our current image
  • perl (perldebug)
  • python
• Dumps you in bash
  • golang

Proposal

1. Add a shell script that will detect if you are running pwsh (or similar), if so, send it to bash, otherwise run pwsh -nologo -l -c "$@"
2. set that script as the ENTRYPOINT
3. SetCMD to [ "pwsh" ]

• Maintain the most features
• Achieves the goals of the best practices

New Scenario

Note the lack of pwsh -c.

docker  run --rm  mcr.microsoft.com/powershell:7.1.0-preview.1-ubuntu-18.04 '$psversiontable.PSVersion.ToString()'

Will continue to work

docker  run --rm  mcr.microsoft.com/powershell:7.1.0-preview.1-ubuntu-18.04 pwsh -c '$psversiontable.PSVersion.ToString()'

Breaking change

Scenario - Running PowerShell using container

Working command with current images

docker  run --rm  mcr.microsoft.com/powershell:7.1.0-preview.1-ubuntu-18.04 /opt/microsoft/powershell/7-preview/pwsh -c '$psversiontable.PSVersion.ToString()'

Scenario - Expected result (with current image)

7.1.0-preview.1

Actual result (with private build with change)

ParserError: 
Line |
   1 |  7.1.0-preview.1
     |       ~~~~~~~~
     | Unexpected token '-preview' in expression or statement.

Workaround

Use --% to prevent the new PowerShell entrypoint from parsing the command.

docker  run --rm  mcr.microsoft.com/powershell:7.1.0-preview.1-ubuntu-18.04 /opt/microsoft/powershell/7-preview/pwsh --% -c '$psversiontable.PSVersion.ToString()'

Entry point script - Ubuntu

#!/bin/sh
set -e

# first arg is `pwsh` or similar
if ! { [ $1 = 'pwsh' ] || [ $1 = 'pwsh-preview' ]; }; then
 set -- pwsh -nologo -l -c "$@"
fi

exec "$@"