Fix#977 upgrade go mod dependencies

[nonetheless]

upgrade go mod dependencies, direct update:

• github.com/moby/buildkit v0.6.3
• github.com/google/go-containerregistry v0.0.0-20200115214256-379933c9c22b

Fixes #977

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[cvgw]

There are a lot of dependencies changed in this PR; enough to make it high risk IMO.

I would strongly prefer that a single dependency (and any absolutely necessary related dependencies) are updated.

This also seem like it might have overlap with #957

[tejal29]

@nonetheless is this PR still needed? #957 is already merged.
Can you rebase this PR to update just buildkit?

[antechrestos]

I've redone this job with current master branch, by just upgrading moby/buildkit to v0.6.3 (2 days later) and go-containerregistry to v0.0.0-20200115214256-379933c9c22b (one month minus end of year holiday) and it gives the following diff of go.mod

Then rm -rf vendor && go mod tidy && go mod vendor && git add vendor gives 1063 modified files.

BTW, I hope there is a go mod tidy verification because the suggested modified files fails as it brings
invalid pseudo-version errors.