fix: pass the --load flag with buildx build to make result img available

[jjmachan]

Description

loads the final image build into system-wide docker. This fixes the docker image missing error when the docker buildx builder is a container-based one. @ssheng had experienced this issue on his system closes:

[codecov[bot]]

Codecov Report

Merging #187 (dcf5ef2) into main (a3471f1) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #187   +/-   ##
=======================================
  Coverage   59.05%   59.05%           
=======================================
  Files          24       24           
  Lines        1138     1138           
=======================================
  Hits          672      672           
  Misses        466      466           

Flag	Coverage Δ
unit-tests	59.05% <ø> (ø)

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Impacted Files	Coverage Δ
bentoctl/docker_utils.py	24.65% <ø> (ø)

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[ssheng]

Looks like there are lint and format error. Please address and merge. @jjmachan