docker: fix example override
docker: fix example override

- Removes incorrect warning about Alpine images
- Shows examples for using locally-built images
- Fixes the environment vars for S3 tests
- Removes unnecessary and confusing volume examples

uoregon-libraries/rais-image-server,uoregon-libraries/rais-image-server,uoregon-libraries/rais-image-server,uoregon-libraries/rais-image-server