Allow Local Images for Docker Storage

[joshmeek]

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• adds new tests (if appropriate)
• updates CHANGELOG.md (if appropriate)
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What does this PR change?

Closes #722

Why is this PR important?

Sometimes users want to build custom base images before adding their flows. This allows for it by skipping the pull step if user specifies local_image=True

[joshmeek]

Also, did verify locally that this works (truncated for ease of reading):

# Building local image
from io import BytesIO

from docker import APIClient

dockerfile = '''
# Shared Volume
FROM python:3.6
RUN ls
'''

f = BytesIO(dockerfile.encode('utf-8'))

cli = APIClient(base_url='unix://var/run/docker.sock')

response = [line for line in cli.build(
    fileobj=f, rm=True, tag='mytest/local'
)]

# Using local image in Docker storage
from prefect import Flow
from prefect.environments.storage import Docker

st = Docker(registry_url=...", base_image="mytest/local:latest", local_image=True)

f = Flow("my_local_test", storage=st)

f.deploy(project_name="...")

# Output from deployment showing it was used
Step 1/9 : FROM mytest/local:latest
 ---> 06ddeb338f8d
...

[codecov]

Codecov Report

Merging #1052 into master will increase coverage by <.01%.
The diff coverage is 100%.

[cicdw]

Whoa this is so much simpler than I imagined it would be - awesome!!