Is it still possible to build locally with Docker?

Hi,

Thanks for all the latest work on this project, it's very much appreciated!

I'm wondering if the local Docker build still works. I tried to run it on my fork and while it does seem like it uses the `depot_tools` to build v8, which I assume is the new workflow, it fails here:

```
/usr/bin/python -u tools/mb/mb.py gen -f infra/mb/mb_config.pyl -m developer_default -b arm64.release out.gn/arm64.release

Writing """\
is_debug = false
target_cpu = "x64"
v8_target_cpu = "arm64"
""" to /v8build/v8/out.gn/arm64.release/args.gn.

/v8build/v8/buildtools/linux64/gn gen out.gn/arm64.release --check
-> returned 1
ERROR at //gni/v8.gni:8:1: Can't load input file.
import("//build/split_static_library.gni")
^----------------------------------------
Unable to load:
/v8build/v8/build/split_static_library.gni
I also checked in the secondary tree for:
/v8build/v8/build/secondary/build/split_static_library.gni
See //BUILD.gn:18:1: whence it was imported.
import("gni/v8.gni")
^------------------
GN gen failed: 1
return self._options.func()
File "./tools/dev/v8gen.py", line 169, in cmd_gen
gn_outdir,
File "./tools/dev/v8gen.py", line 211, in _call_cmd
stderr=subprocess.STDOUT,
File "/usr/lib/python2.7/subprocess.py", line 574, in check_output
raise CalledProcessError(retcode, cmd, output=output)
subprocess.CalledProcessError: Command '['/usr/bin/python', '-u', 'tools/mb/mb.py', 'gen', '-f', 'infra/mb/mb_config.pyl', '-m', 'developer_default', '-b', 'arm64.release', 'out.gn/arm64.release']' returned non-zero exit status 1
The command '/bin/sh -c ./tools/dev/v8gen.py ${build_platform} -vv' returned a non-zero code: 1
```