doc: the `--extra-build-args` flag is ~ misleading when using `build-webapp`

[zramsay]

For example, the website needs this env var on image build:

laconic-so build-webapp --source-repo laconic.com/ --extra-build-args "--build-arg NEXT_PUBLIC_DATOCMS_BYPASS_TYPE='local_json'"

and although this will build without erroring, buried in the output is the following:
[Warning] One or more build-args [NEXT_PUBLIC_DATOCMS_BYPASS_TYPE] were not consumed

a quick google says I should modify the Dockerfile, which wouldn't make sense since build-webapp is using an image that we don't want to modify

the solution was to modify package.json like so:

-    "build": "next build",
+    "build": "NEXT_PUBLIC_DATOCMS_BYPASS_TYPE=local_json next build",

assigning myself as a documentation issue