dockerutil: fix docker version parsing

AFAICT previously the function was parsing the Docker *client* version, since
the output of `docker version` has the client version first, which gets picked
up by the `re.FindStringSubmatch()` since it finds the _first_ match:

```
$ docker version
Client: Docker Engine - Community
 Version:           29.6.2
 API version:       1.55
 Go version:        go1.26.5
 Git commit:        dfc4efb
 Built:             Thu Jul 16 16:12:34 2026
 OS/Arch:           linux/amd64
 Context:           default

Server: Docker Engine - Community
 Engine:
  Version:          29.6.2
  API version:      1.55 (minimum version 1.40)
  Go version:       go1.26.5
  Git commit:       3d80467
  Built:            Thu Jul 16 16:12:34 2026
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          v2.2.6
  GitCommit:        11ce9d5f3c68c941867e82890e93e815c1304f1b
 runc:
  Version:          1.3.6
  GitCommit:        v1.3.6-0-g491b69ba
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

```

PiperOrigin-RevId: 966681719