Readme: Fix command to build with Docker on Windows

[avgrad]

The command from the README.md for building with Docker does not work on Windows.

$(pwd) is not valid. Neither in CMD nor in PowerShell.

This PR adds a separate listing and command for PowerShell with ${pwd} that works (braces instead of parentheses).

[Carlgo11]

$(pwd) could just be replaced with ./ for it to work on all platforms.

[Carlgo11]

As the requested changes haven't been resolved yet I'm going to label this PR as inactive.
⚠️ Unless there's activity within the next 7 days I'll close the PR.

[avgrad]

$(pwd) could just be replaced with ./ for it to work on all platforms.

docker run -p 4000:4000 -v ./:/twofactorauth 2factorauth/twofactorauth

I tried this on Windows Powershell and CMD, both resulting in the following error:

docker: Error response from daemon: create .: volume name is too short, names should be at least two alphanumeric characters.
See 'docker run --help'.

[Carlgo11]

Ah okay.
I guess ${PWD} works in PS?
If so, replace $(pwd) with ${PWD} as it should be available cross platform.

Although the entire ordeal might be a bit of a waste due to #7091 removing that part of the README completely 🤷‍♂️

[avgrad]

Yes for Windows PowerShell ${pwd} works.
I can't test ${pwd} on Linux and Mac.
I merged the commands to use ${pwd} for all platforms.