Hi anatwick and welcome to the Mattermost forums!
The sed command only works if you have the nginx config file available. In your case, you said you’re using an nginx container and the client_max_body_size parameter needs to be changed in the container’s nginx config.
What container are you using and how do you start it? Maybe there’s an environment variable or the possibility to add configuration options from “the outside” so that the config gets applied properly.
If you’re not using the Mattermost-integrated nginx container you most likely also had to create a configuration file for the Mattermost VHOST there - the server section in this config file would also be a good location to override the default client_max_body_size of the container’s nginx setup.