[build]: support specifying builder mount point and workdir

[ishidawataru]

- What I did

support buildimage repo to be a git submodule

- How I did it

If we put this repo as a git submodule of other git repo, make all fails because sonic_get_version() doesn't work inside a build container.

Inside sonic_get_version(), git describe commands are executed. To make these command work, git command need to access the .git directory which is not mounted in a build container.

This commit makes a build container mount a parent git directory to give access to the .git directory if this repo is a git submodule.

- How to verify it

make this repo git submodule and run make all

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[lguohan]

the solution is a little bit complicated, why not expose the option to allow user to mount any place into the docker?

[ishidawataru]

The directory we need to mount and the working directory is different in git submodule case.

For example, if sonic-buildimage is a git submodule of parent_repo, we need to mount parent_repo and set parent_repo/sonic-buildimage as the working directory.

With your suggestion, should I add two options, one for the directory to mount and the other for the working directory?

[lguohan]

yes, that will do.

[ishidawataru]

@lguohan followed your suggestion. PTAL.

[lguohan]

can you change the title and commit message?

[ishidawataru]

@lguohan How should I change them?

[stcheng]

@ishidawataru you could force push to update the commit message

[ishidawataru]

@stcheng I meant the sentence of the title and the commit message.

Current commit message is the below.

[build]: support specifying builder mount point and workdir

This can be used when sonic-buildimage repo is a git submodule of other
repo.

e.g) if sonic-buildimage is a git submodule of "parent" repo and
sonic-buildimage is just under the top directory of "parent" repo, we
can build sonic-buildimage like below

$ cd parent/sonic-buildimage
$ DOCKER_BUILDER_MOUNT=`realpath ../`:`realpath ../` \
  DOCKER_BUILDER_WORKDIR=`pwd` make all

Signed-off-by: Wataru Ishida <ishida@nel-america.com>

I'm not sure how the title/message should be for this commit.

I know about force push, thanks :)