`COPY ./.git X` command errors in buildx `v0.13.1` about buildkit

[tonistiigi]

#4776

[raylu]

I don't think the above fixes it in general. we have a build context with an absolute path (/src.git)
so when we build

$ cat Dockerfile 
FROM alpine

COPY --from=src file /

this still breaks. it used to work in docker 24.0.5:

$ docker buildx version
github.com/docker/buildx v0.11.2 9872040b6626fb7d87ef7296fd5b832e8cc2ad17

but in docker 24.0.9 and 25.0.3,

 $ docker buildx version
github.com/docker/buildx v0.13.1 788433953af10f2a698f5c07611dddce2e08c7a0
~ $ docker buildx build --build-context src=/src.git .
[+] Building 0.2s (4/4) FINISHED                                                                                   docker:default
 => [internal] load local bake definitions                                                                                   0.0s
 => => reading Bakefile 246B / 246B                                                                                          0.0s
 => [internal] load build definition from Dockerfile                                                                         0.0s
 => => transferring dockerfile: 73B                                                                                          0.0s
 => [internal] load .dockerignore                                                                                            0.0s
 => => transferring context: 2B                                                                                              0.0s
 => CANCELED [internal] load metadata for docker.io/library/alpine:latest                                                    0.0s
Dockerfile:3
--------------------
   1 |     FROM alpine
   2 |     
   3 | >>> COPY --from=src file /
   4 |     
--------------------
ERROR: failed to solve: invalid context specifier /src.git for src

the fix in eb41916 only works if the path starts with ./ or ../, which our absolute paths don't