chore: copy bundle-size to root (#877)

Docker actions get their workspace overridden as `/docker/workspace`.  Your project files are copied to that folder \*after* your docker file is run but \*before* the `CMD` directive is executed, so you can't copy anything to that folder and expect it to still be there.

The contents of `/docker/workspace` are the project, not the gh action, so `dist/index.js` does not exist.

We don't ship gh actions with aegir so you can't use `/docker/workspace/node_modules/aegir/actions/bundle-size/dist/index.js` either.

Also, you can't use slashes in branch names for gh actions as they get copied onto the filesystem then the directory structure doesn't match up.

The working directory of the docker image is `/docker/workspace` so we can copy the bundle size index.js to the root and run it from there but check the size of the current project due to the CWD being set to the docker workspace.

This branch is building
 * a CJS module here: multiformats/js-multiaddr#207
 * a ESM module here: achingbrain/uint8arrays#26

Author: achingbrain

actions/bundle-size/Dockerfile

@@ -1,5 +1,5 @@
 # Just enough docker until github gets a new node16 runner
 # see: https://github.com/actions/runner/issues/772
 FROM node:16-alpine
-WORKDIR /usr/src/app
-CMD [ "node", "dist/index.js" ]
+COPY ./dist/index.js /bundle-size.js
+CMD [ "node", "/bundle-size.js" ]