Send better user-agent values (and got config changes) (#7309)

* expose fetchLimitBytes/userAgent in got-config module

* export a function not a factory

* send better user-agent values

- add userAgentBase setting
- send short SHA in user agent on heroku
- set a version (tag or short SHA) in Dockefile and use
  it to report server version in UA for docker users

* add a comment explaining fileSize

Co-authored-by: repo-ranger[bot] <39074581+repo-ranger[bot]@users.noreply.github.com>

Dockerfile

@@ -20,6 +20,10 @@ RUN npm cache clean --force
 
 # Use multi-stage build to reduce size
 FROM node:16-alpine
+
+ARG version=dev
+ENV DOCKER_SHIELDS_VERSION=$version
+
 # Run the server using production configs.
 ENV NODE_ENV production