$ sh -x test.sh
+ mkdir a b
+ echo hello from a
+ echo hello from b
+ dd if=/dev/urandom of=b/largefile bs=1M count=1
1+0 records in
1+0 records out
1048576 bytes (1,0 MB, 1,0 MiB) copied, 0,00433338 s, 242 MB/s
+ cat
+ trap rm -rf a b Dockerfile EXIT
+ ls -laR .
.:
total 24
drwxrwxr-x  4 kohtala kohtala 4096 tammi  15 18:33 .
drwxrwxr-x 49 kohtala kohtala 4096 tammi  15 17:34 ..
drwxrwxr-x  2 kohtala kohtala 4096 tammi  15 18:33 a
drwxrwxr-x  2 kohtala kohtala 4096 tammi  15 18:33 b
-rw-rw-r--  1 kohtala kohtala  109 tammi  15 18:33 Dockerfile
-rw-rw-r--  1 kohtala kohtala  558 tammi  15 18:33 test.sh

./a:
total 12
drwxrwxr-x 2 kohtala kohtala 4096 tammi  15 18:33 .
drwxrwxr-x 4 kohtala kohtala 4096 tammi  15 18:33 ..
-rw-rw-r-- 1 kohtala kohtala   13 tammi  15 18:33 file

./b:
total 1036
drwxrwxr-x 2 kohtala kohtala    4096 tammi  15 18:33 .
drwxrwxr-x 4 kohtala kohtala    4096 tammi  15 18:33 ..
-rw-rw-r-- 1 kohtala kohtala      13 tammi  15 18:33 file
-rw-rw-r-- 1 kohtala kohtala 1048576 tammi  15 18:33 largefile
+ docker buildx build --load --build-context context=a --no-cache -t a .
#0 building with "buildkit-dev" instance using docker-container driver

#1 [internal] load build definition from Dockerfile
#1 transferring dockerfile: 146B done
#1 DONE 0.0s

#2 [auth] library/alpine:pull token for registry-1.docker.io
#2 DONE 0.0s

#3 [context context] load .dockerignore
#3 transferring _context: 2B done
#3 DONE 0.0s

#4 [internal] load metadata for docker.io/library/alpine:latest
#4 DONE 0.8s

#5 [internal] load .dockerignore
#5 transferring context: 2B done
#5 DONE 0.0s

#6 [context context] load from client
#6 transferring _context: 44B done
#6 DONE 0.0s

#7 [stage-0 1/2] FROM docker.io/library/alpine:latest@sha256:865b95f46d98cf867a156fe4a135ad3fe50d2056aa3f25ed31662dff6da4eb62
#7 resolve docker.io/library/alpine:latest@sha256:865b95f46d98cf867a156fe4a135ad3fe50d2056aa3f25ed31662dff6da4eb62 0.0s done
#7 CACHED

#8 [stage-0 2/2] RUN --mount=from=context,target=/context <<NUR (cat /context/file...)
#8 0.071 hello from a
#8 DONE 0.1s

#9 exporting to oci image format
#9 exporting layers 0.1s done
#9 exporting manifest sha256:1e12f7bddb93dd3f36ef3cef9adb2a2b0ee5fd62c34e70eb92a475cbaa04d999 0.0s done
#9 exporting config sha256:1f33eed3b2e20b39981f097bb58a4ac389387bb7ed68d3aaa50b1abcea5ac4c4 0.0s done
#9 sending tarball
#9 sending tarball 0.3s done
#9 DONE 0.4s

#10 importing to docker
#10 DONE 0.0s
+ docker buildx build --load --build-context context=b --no-cache -t b .
#0 building with "buildkit-dev" instance using docker-container driver

#1 [internal] load build definition from Dockerfile
#1 transferring dockerfile: 146B done
#1 DONE 0.0s

#2 [context context] load .dockerignore
#2 transferring _context: 2B done
#2 DONE 0.0s

#3 [internal] load metadata for docker.io/library/alpine:latest
#3 DONE 0.2s

#4 [internal] load .dockerignore
#4 transferring context: 2B done
#4 DONE 0.0s

#5 [context context] load from client
#5 transferring _context: 1.05MB 0.0s done
#5 DONE 0.0s

#6 [stage-0 1/2] FROM docker.io/library/alpine:latest@sha256:865b95f46d98cf867a156fe4a135ad3fe50d2056aa3f25ed31662dff6da4eb62
#6 resolve docker.io/library/alpine:latest@sha256:865b95f46d98cf867a156fe4a135ad3fe50d2056aa3f25ed31662dff6da4eb62 0.0s done
#6 CACHED

#7 [stage-0 2/2] RUN --mount=from=context,target=/context <<NUR (cat /context/file...)
#7 0.063 hello from a
#7 DONE 0.1s

#8 exporting to oci image format
#8 exporting layers 0.1s done
#8 exporting manifest sha256:446979d0ea309400583ced6c2e62953795fe4828151dfbf177d9861ffffead3d 0.0s done
#8 exporting config sha256:1ad64adad6e9f3465a9a38932539dd383e96fe14213ae7c8da2d1d2b5ea1af71 0.0s done
#8 sending tarball
#8 sending tarball 0.2s done
#8 DONE 0.4s

#9 importing to docker
#9 DONE 0.0s
+ docker run --rm a cat /file
hello from a
+ docker run --rm b cat /file
hello from a