build: fix devcontainers postCreateCommand permissions, fixes #6187

[stasadev]

The Issue

• vscode - mkdir: cannot create directory ‘/workspaces/d9simple’: Permission denied #6187

How This PR Solves The Issue

Sets directory permissions for /workspaces/d10simple
Removes hard-coded path for /workspaces/ddev (because users can clone the repo to a different directory).

Manual Testing Instructions

git clone https://github.com/ddev/ddev.git my-ddev-copy
code my-ddev-copy

# install Dev Containers extension
# Menu > View > Command Palette... > Dev Containers: Reopen in Container
# wait some time, I had to wait ~10 minutes

ls -l /workspaces
total 8
drwxr-xr-x  7 codespace codespace 4096 May 16 12:40 d10simple
drwxr-xr-x 25 codespace codespace 4096 May 16 11:46 my-ddev-copy

Automated Testing Overview

Related Issue Link(s)

Release/Deployment Notes

[rfay]

This works fine for me in codespaces, thanks!