For the sake of completeness, here is how I solved it.
I'm not very proud of the solution but it works.
In the emailer plugin, I added this bit to mailOptions variable.
Now I need to be very careful not to update the plugin and overwrite the index.js file
var mailOptions = {
from: data.from,
to: data.to,
html: data.html.replaceAll ('src="/nodebb/','src="http://dash/nodebb/'),
text: data.plaintext,
// subject: data.subject
subject: combinedTopic
};
