dockerfile: validate order when linking stages

[tonistiigi]

Dockerfile stages always needed to be in order (you can not refer to a stage that is defined after your current command), but the behavior for incorrect order was undefined instead of consistently erroring.

Previously copying from a stage defined later would usually cause an error about a missing source file that could have been quite confusing. When the source path was "/" then the behavior could have been worse and resulted in COPY passing without error, without any files actually being copied (or only files from base image being copied).

This validates the order when on COPY and mount dispatch, returns proper error with correct source code location.

Dockerfile:5
--------------------
   3 |
   4 |     FROM busybox AS target
   5 | >>> COPY --from=build / /out
   6 |
   7 |     FROM alpine AS build
--------------------
error: failed to solve: cannot copy from stage "build", it needs to be defined before current stage "target"

[crazy-max]

Nice! This will be easier to grasp.

This test seems to fail after this change when using implicit from?:

=== FAIL: frontend/dockerfile TestIntegration/TestMultiStageImplicitFrom/worker=containerd-rootless/frontend=client (0.62s)
    dockerfile_test.go:3694: 
        	Error Trace:	/src/frontend/dockerfile/dockerfile_test.go:3694
        	            				/src/util/testutil/integration/run.go:93
        	            				/src/util/testutil/integration/run.go:207
        	Error:      	Received unexpected error:
        	            	lstat /tmp/buildkit-mount4017488364/etc/passwd: no such file or directory
        	            	github.com/moby/buildkit/util/stack.Enable
        	            		/src/util/stack/stack.go:77
        	            	github.com/moby/buildkit/util/grpcerrors.FromGRPC
        	            		/src/util/grpcerrors/grpcerrors.go:198
        	            	github.com/moby/buildkit/util/grpcerrors.UnaryClientInterceptor
        	            		/src/util/grpcerrors/intercept.go:41
        	            	google.golang.org/grpc.(*ClientConn).Invoke
        	            		/src/vendor/google.golang.org/grpc/call.go:35
        	            	github.com/moby/buildkit/api/services/control.(*controlClient).Solve
        	            		/src/api/services/control/control.pb.go:2234
        	            	github.com/moby/buildkit/client.(*Client).solve.func2
        	            		/src/client/solve.go:274
        	            	golang.org/x/sync/errgroup.(*Group).Go.func1
        	            		/src/vendor/golang.org/x/sync/errgroup/errgroup.go:75
        	            	runtime.goexit
        	            		/usr/local/go/src/runtime/asm_amd64.s:1650
        	            	failed to solve
        	            	github.com/moby/buildkit/client.(*Client).solve.func2
        	            		/src/client/solve.go:290
        	            	golang.org/x/sync/errgroup.(*Group).Go.func1
        	            		/src/vendor/golang.org/x/sync/errgroup/errgroup.go:75
        	            	runtime.goexit
        	            		/usr/local/go/src/runtime/asm_amd64.s:1650

[bperkins24]

This appears to have broken using a command like this in a base image:

ONBUILD RUN --mount=type=cache,target=/root/.cache/pip ...

When building the image referencing the base image, the error is failed to solve: cannot mount from stage "scratch" to "/root/.cache/pip", stage needs to be defined before current command

Pinning the Dockerfile referencing the base image to # syntax=docker/dockerfile:1.6 works but is not a viable long term solution. How do I go about submitting a bug report and getting this resolved?