[docker] Add /dev/fuse for mounting containers in docker

[romilbhardwaj]

Storage mounting was not working when running inside a docker container on the new provisioner. This PR fixes it and adds smoke tests.

Minimal repro:

resources:
  cloud: gcp
  image_id: docker:ubuntu:latest

file_mounts:
  /mymnt:
    source: s3://fah-public-data-covid19-cryptic-pockets #romileubucket2
    mode: MOUNT
  /mynotebooks:
    source: gs://romilb-notebooks/ #romileubucket2
    mode: MOUNT

setup: |
  echo hi

run: |
  ls -l /mynotebooks
  ls -l /mymnt
  echo run > /mynotebooks/hellotest.txt

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Smoke - pytest tests/test_smoke.py::test_docker_storage_mounts