Magento 2 local Docker - running less:theme fails for file not found, but file is present

Question

I finally have a local docker setup for my Magento 2 development set up. It works fine everywhere, except for when I run grunt less:MyTheme. When I do that I get this error

Running "less:MyTheme" (less) task
 FileError: 'source/_extend.less' wasn't found. Tried - pub/static/frontend/MyCompany/MyTheme/nl_NL/css/source/_extend.less,pub/static/frontend/MyCompany/MyTheme/nl_NL/css/source/_extend.less,source/_extend.less in pub/static/frontend/MyCompany/MyTheme/nl_NL/css/styles-m.less on line 91, column 1:
 90 
 91 @import 'source/_extend.less';
 92
Warning: Error compiling pub/static/frontend/MyCompany/MyTheme/nl_NL/css/styles-m.less Use --force to continue.

Aborted due to warnings.

After this all CSS is broken. But the file in question, pub/static/frontend/MyCompany/MyTheme/nl_NL/css/source/_extend.less does exist with correct permissions and all. I can't figure out what happens.

As this is an exact copy of my dev server install, where this command works without problems, I figure this has something to do with the Docker setup or something, but I have no clue where to look.

Some details and things I already tried;

• the .htaccess files are all present where most answers point to
• static content been deployed without problems
• the file said missing, does exist on the path it tries(?!)
• use this docker environment
• after I run magento setup:di:compile the css does work again
• running nginx

What do these symptons point to?

Answer 1

Actually, when compiling to the pub/static files, the original Less files that are used are the ones in your app/design folder. In this case:

app/design/frontend/MyCompany/MyTheme/web/css/source/_extend.less