This allows clients to specify that LLB states should be grouped in
progress output under a custom name. Status updates for all vertexes in
the group will show up under a single vertex in the output.
The intended use cases are for Dockerfile COPY's that use MergeOp as a
backend and for grouping some other internal vertexes during frontend
builds.
Signed-off-by: Erik Sipsma <erik@sipsma.dev>
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