allow sanic build to pull local images if they exist

[tonzgao]

Dockerfile FROM db command works with docker build but not sanic build for local images

/home/tonz/projects/fast-flier-deals/services/migrator
[2019-10-06 17:23:16.07380992 -0400 EDT] transferring dockerfile: 178.00B
[2019-10-06 17:23:16.664207514 -0400 EDT] transferring context: 2.00B
[2019-10-06 17:23:19.592558928 -0400 EDT] [1/3] FROM docker.io/library/db:latest
[2019-10-06 17:23:19.592608417 -0400 EDT] [2/3] ADD requirements.txt /tmp
[2019-10-06 17:23:19.592621582 -0400 EDT] [3/3] RUN pip install -r /tmp/requirements.txt && rm /tmp/requirements.txt
[2019-10-06 17:23:19.592676532 -0400 EDT] [1/3] FROM docker.io/library/db:latest
[2019-10-06 17:23:19.919727448 -0400 EDT] pull access denied, repository does not exist or may require authorization: server message: insufficient_scope: authorization failed: LAYERID=sha256:ccb5ad2d63d98d3b08defe021f5a2a88c5aa993258a983c15670ef347f519da2
[2019-10-06 17:23:19.919781622 -0400 EDT] [1/3] FROM docker.io/library/db:latest
[2019-10-06 17:23:19.918292165 -0400 EDT] resolve docker.io/library/db:latest 0B
[2019-10-06 17:23:19.920876694 -0400 EDT] FAILED: failed to solve: rpc error: code = Unknown desc = failed to build LLB: failed to load cache key: pull access denied, repository does not exist or may require authorization: server message: insufficient_scope: authorization failed
[2019-10-06 17:23:19.920960577 -0400 EDT] Build failed! failed to solve: rpc error: code = Unknown desc = failed to build LLB: failed to load cache key: pull access denied, repository does not exist or may require authorization: server message: insufficient_scope: authorization failed

without this it is hard to do local baseimages or libraries (like db)

solution probably involves some sort kind of dependency checking for build ordering