tramp-container doesn't follow `ENV PATH` in docker file

[ParetoOptimalDev]

Hi, all. I'm having some issues with using docker container and have an
idea for improving it.

Firstly, I'm familiar with
https://www.gnu.org/software/emacs//manual/html_node/tramp/Remote-programs.html
and `tramp-own-remote-path`, but they aren't sufficient for seamlessly
dropping into a docker container and using binaries inside of it.

Here's an example with emacs 30 using `emacs -Q`. First the code:

```
(require 'tramp)
(message "")

(message "using the docker haskell image")
(message (concat "path should be: 
""/root/.cabal/bin:/root/.local/bin:/opt/ghc/9.4.2/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"))

(message "show that using tramp-own-remote-path isn't sufficient")
(let ((docker-container-id (string-trim (shell-command-to-string "docker run 
-it --rm --detach haskell"))))
  (let ((default-directory (concat "/docker:" docker-container-id ":"))
        (tramp-remote-path '(tramp-own-remote-path)))
        (message docker-container-id)
        (message (concat "path is: " (truncate-string-to-width 
(eshell-command-result "echo $PATH") 100)))))
```

Here are it's results:

```
using the docker haskell image
path should be: 
/root/.cabal/bin:/root/.local/bin:/opt/ghc/9.4.2/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
show that using tramp-own-remote-path isn’t sufficient
69ae6adb42d95fd9ebef5738218217d822ee2977956b61227b43eb8ccb1401d4
Tramp: Opening connection nil for 
69ae6adb42d95fd9ebef5738218217d822ee2977956b61227b43eb8ccb1401d4 using 
docker...done
path is: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
```

This means that when you:

- open eshell
- navigate to
- `/docker:69ae6adb42d95fd9ebef5738218217d822ee2977956b61227b43eb8ccb1401d4:`
- run `ghc`

You'll be met with an error that `ghc` doesn't exist in the Haskell
docker container.

It seems that the typical way for docker containers to specify PATH uses
`ENV PATH` so it seems good to support it (if that's not already
implemented).

I noticed when using docker inspect on the container that it contains
the PATH set by the docker file.

Perhaps tramp could simply use that for the docker container method?

Thanks.