Fixes #812, #778, #573.

In issue [#573](https://github.com/pulumi/pulumi-docker/issues/573), users would
sometimes report the image push fails with an error "the image does not exist
locally with the tag". This is likely due to Docker Engine RPCs overflowing the
default `bufio.Scanner` maximum buffer size of 64k. Replacing the scanner with a
`json.Decoder` enables us to handle large responses. When the image build fails,
users should now see a warning or error before the provider pushes the built
image.

In issue [#812](https://github.com/pulumi/pulumi-docker/issues/812), users would
see the update operation on the Image resource succeed, but the output
`repoDigest` would not change. In this PR, if for any reason the image fails to
build, or update the Docker Engine's image store, push, or we detect
inconsistency between the image ID returned by the `ImageBuild` RPC and what we
expected, we fail the resource operation with an error.

In issue [#778](https://github.com/pulumi/pulumi-docker/issues/778), a repo
digest would be chosen that doesn't match the name (sans tag) of the image
pushed. As part of the additional checks, we fix this and ensure the normalized
name of the repo digest matches the returned repo digest.

Additionally, in case there is a data race between `ImageBuild` and `ImagePush`,
we extract the expected image IDs and repo digests from those operations and
compare to the image store. As this is a net-new check and we could not
reproduce a repo digest mismatch, this is a warning.

In a follow-up (#846) we may consider removing some of the defensive programming
in this change.