feat: Support scanning the local Docker daemon's image list

[achimnol]

resolves #1089

Note that I don't add --local option to the client CLI's admin image rescan command because this is intended for local all-in-one development setups where you can directly access the manager CLI command mgr image rescan.

Known restriction

When starting a new session via CLI, you must prepend "index.docker.io/library/" to your locally built image names if they don't have hostname/path prefixes.

Examples:

• bai-python:3.11 -> index.docker.io/library/bai-python:3.11
• localdev/bai-python:3.11 -> index.docker.io/localdev/bai-python:3.11

You also need to add index.docker.io to the allowed registry list of the default domain.

[achimnol]

Now you can build a simple Dockerfile like this:

FROM python:3.11

# Install ipython kernelspec
RUN python3.11 -m pip install ipython ipykernel
RUN python3.11 -m ipykernel install --display-name "Python 3.11 on Backend.AI" && \
    cat /usr/local/share/jupyter/kernels/python3/kernel.json

LABEL ai.backend.kernelspec=1 \
      ai.backend.features="uid-match batch query" \
      ai.backend.base-distro="ubuntu18.04" \
      ai.backend.runtime-type="python" \
      ai.backend.runtime-path="/usr/local/bin/python3"

by:

docker build -t bai-python:3.11 -f python.Dockerfile .

and run it locally on a development setup as follows:

./backend.ai mgr image rescan --local
./backend.ai mgr start-server

./backend.ai ag start-server

source env-local-admin-api.sh
./backend.ai admin domain update \  # required for the first time only
  --allowed-docker-registries cr.backend.ai \  # depends on your setup
  --allowed-docker-registries index.docker.io \
  default
./backend.ai run --rm -c 'import sys;print("hello", sys.version_info)' index.docker.io/library/bai-python:3.11

Remaining TODO: Update the existing test suite

[achimnol]

TODO: prevent duplication of pulled images? (maybe checking the digest in the database)