dockerd reliably segfault with .dockerignore and copy

### Description

Build fails because dockerd segfaults. This happens with a very specific invalid .gitignore and some copy statements in Dockerfile.

Minimal config to reproduce is below.

Tried with 28.5.2, 29.2.1, 29.3.0.

### Reproduce

.dockerignore contains:
```
[Local-Only]/
```
(which is not a valid pattern and causes a validation error, except in these circumstances)

Dockerfile contains:
```
FROM python:3.12-slim

WORKDIR /app

COPY a a
COPY b b

```

Build it:
`docker build .`

Build fails with:
`ERROR: failed to build: failed to receive status: rpc error: code = Unavailable desc = error reading from server: EOF`

The log shows segfault in dockerd causing service to restart:
```
journalctl -u docker.service -f
dockerd[423967]: panic: runtime error: invalid memory address or nil pointer dereference
dockerd[423967]: [signal SIGSEGV: segmentation violation code=0x1 addr=0x90 pc=0x557e0f7ebf80]
dockerd[423967]: goroutine 1241 [running]:
dockerd[423967]: regexp.(*Regexp).doExecute(0x557e11b285a0?, {0x0?, 0x0?}, {0x0?, 0x557e11922650?, 0x557e11922650?}, {0xc0009d3db0?, 0xc000061778?}, 0x557e0f6d0d99?, 0x0, ...)
dockerd[423967]:         /usr/local/go/src/regexp/exec.go:527 +0x80
dockerd[423967]: regexp.(*Regexp).doMatch(...)
dockerd[423967]:         /usr/local/go/src/regexp/exec.go:514
dockerd[423967]: regexp.(*Regexp).MatchString(...)
dockerd[423967]:         /usr/local/go/src/regexp/regexp.go:527
dockerd[423967]: github.com/moby/patternmatcher.(*Pattern).match(0x557e11922650?, {0xc0009d3db0, 0x1})
dockerd[423967]:         /root/build-deb/engine/vendor/github.com/moby/patternmatcher/patternmatcher.go:334 +0x26b
dockerd[423967]: github.com/moby/patternmatcher.(*PatternMatcher).MatchesOrParentMatches(0xc000d761e0, {0xc0009d3db0, 0x1})
dockerd[423967]:         /root/build-deb/engine/vendor/github.com/moby/patternmatcher/patternmatcher.go:142 +0xda
dockerd[423967]: github.com/moby/buildkit/frontend/dockerfile/dockerfile2llb.validateCopySourcePath({0xc0009d3db0, 0x1}, 0xc0000621f8)
dockerd[423967]:         /root/build-deb/engine/vendor/github.com/moby/buildkit/frontend/dockerfile/dockerfile2llb/convert.go:2023 +0x55
dockerd[423967]: github.com/moby/buildkit/frontend/dockerfile/dockerfile2llb.dispatchCopy(_, {{{0xc0009d3dc0, 0x1}, {0xc0009b5d10, 0x1, 0x1}, {0x0, 0x0, 0x0}}, {0x0, ...}, ...})
dockerd[423967]:         /root/build-deb/engine/vendor/github.com/moby/buildkit/frontend/dockerfile/dockerfile2llb/convert.go:1607 +0xd5c
dockerd[423967]: github.com/moby/buildkit/frontend/dockerfile/dockerfile2llb.dispatch(_, {{_, _}, {_, _, _}, _}, {0xc000aab6a0, {0x557e1214c560, 0xc0007b79e0}, ...})
dockerd[423967]:         /root/build-deb/engine/vendor/github.com/moby/buildkit/frontend/dockerfile/dockerfile2llb/convert.go:1004 +0xafb
dockerd[423967]: github.com/moby/buildkit/frontend/dockerfile/dockerfile2llb.toDispatchState({_, _}, {_, _, _}, {{0xc000d5c8d0, {0x0, 0x0}, {0x0, 0x0}, ...}, ...})
dockerd[423967]:         /root/build-deb/engine/vendor/github.com/moby/buildkit/frontend/dockerfile/dockerfile2llb/convert.go:731 +0x3926
dockerd[423967]: github.com/moby/buildkit/frontend/dockerfile/dockerfile2llb.Dockerfile2LLB({_, _}, {_, _, _}, {{0xc000d5c8d0, {0x0, 0x0}, {0x0, 0x0}, ...}, ...})
dockerd[423967]:         /root/build-deb/engine/vendor/github.com/moby/buildkit/frontend/dockerfile/dockerfile2llb/convert.go:90 +0x65
dockerd[423967]: github.com/moby/buildkit/frontend/dockerfile/builder.Build.func6({0x557e121613e0, 0xc000cfd590}, 0x0, 0x557e0f64accb?)
dockerd[423967]:         /root/build-deb/engine/vendor/github.com/moby/buildkit/frontend/dockerfile/builder/build.go:136 +0xfe
dockerd[423967]: github.com/moby/buildkit/frontend/dockerui.(*Client).Build.func1()
dockerd[423967]:         /root/build-deb/engine/vendor/github.com/moby/buildkit/frontend/dockerui/build.go:39 +0x71
dockerd[423967]: golang.org/x/sync/errgroup.(*Group).Go.func1()
dockerd[423967]:         /root/build-deb/engine/vendor/golang.org/x/sync/errgroup/errgroup.go:93 +0x50
dockerd[423967]: created by golang.org/x/sync/errgroup.(*Group).Go in goroutine 1136
dockerd[423967]:         /root/build-deb/engine/vendor/golang.org/x/sync/errgroup/errgroup.go:78 +0x95
systemd[1]: docker.service: Main process exited, code=exited, status=2/INVALIDARGUMENT
systemd[1]: docker.service: Failed with result 'exit-code'.
```

### Expected behavior

Error for invalid patter in .dockerignore.

If I remove one of the copy statements from Dockerfile, I get expected error:

`ERROR: failed to build: failed to solve: error from sender: failed to match excludepatterns: syntax error in pattern`

### docker version

```bash
Client: Docker Engine - Community
 Version:           29.3.0
 API version:       1.54
 Go version:        go1.25.7
 Git commit:        5927d80
 Built:             Thu Mar  5 14:25:43 2026
 OS/Arch:           linux/amd64
 Context:           default

Server: Docker Engine - Community
 Engine:
  Version:          29.3.0
  API version:      1.54 (minimum version 1.40)
  Go version:       go1.25.7
  Git commit:       83bca51
  Built:            Thu Mar  5 14:25:43 2026
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          v2.2.2
  GitCommit:        301b2dac98f15c27117da5c8af12118a041a31d9
 runc:
  Version:          1.3.4
  GitCommit:        v1.3.4-0-gd6d73eb8
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0
```

### docker info

```bash
Client: Docker Engine - Community
 Version:    29.3.0
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.31.1
    Path:     /usr/libexec/docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v5.1.1
    Path:     /usr/libexec/docker/cli-plugins/docker-compose

Server:
 Containers: 0
  Running: 0
  Paused: 0
  Stopped: 0
 Images: 10
 Server Version: 29.3.0
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Using metacopy: false
  Native Overlay Diff: true
  userxattr: false
 Logging Driver: json-file
 Cgroup Driver: systemd
 Cgroup Version: 2
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local splunk syslog
 CDI spec directories:
  /etc/cdi
  /var/run/cdi
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 301b2dac98f15c27117da5c8af12118a041a31d9
 runc version: v1.3.4-0-gd6d73eb8
 init version: de40ad0
 Security Options:
  apparmor
  seccomp
   Profile: builtin
  cgroupns
 Kernel Version: 6.1.0-30-cloud-amd64
 Operating System: Debian GNU/Linux 12 (bookworm)
 OSType: linux
 Architecture: x86_64
 CPUs: 4
 Total Memory: 7.767GiB
 Name: pitchomatic
 ID: fbc54e0e-9b06-4058-b8ac-3648f833ee5b
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Experimental: false
 Insecure Registries:
  ::1/128
  127.0.0.0/8
 Live Restore Enabled: false
 Firewall Backend: iptables
```

### Additional Info

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dockerd reliably segfault with .dockerignore and copy #52203

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dockerd reliably segfault with .dockerignore and copy #52203

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