No cigar 
- here is precisely what I did:
Editor the .htaccess file to as shown above
Run docker-compose down
Run docker-compose up -d
At this point, I invoked the wordpress again and tried to install the plugin that gave me the problem before. It still fails with that exceeded size error.
As I inspected the .htaccess file at this point, I saw that it does not include the php_value upload_max_filesize 256M line.So, this “patch on the fly” does not seem to work, or I am not understanding your advice.