This commit introduces the "docker_entrypoint" script that will create
a user with uid/gid matching given `USER_ID` and `GROUP_ID` (or default to
`1000` if not provided).
Fixes#9
This approach works around missing docker feature discussed in
docker/docker#7198 and allow to have executable in the docker container
manipulating files in the shared volume owned by the `USER_ID:GROUP_ID`
The utility script `aosp` has also been updated to automatically
set `USER_ID` and `GROUP_ID` to the value matching the current user
by invoking "docker run" with
```
-e USER_ID=$(id -u) -e GROUP_ID=$(id -g)
```
Finally, the output has also been updated to be more verbose. For example:
```
$ AOSP_VOL=/home/jcfr/Projects/aosp-root/ aosp id
aosp: Checking if /home/jcfr/Projects/aosp-root/aosp exists
aosp: Checking if /home/jcfr/Projects/aosp-root/aosp exists - ok
aosp: Checking if /home/jcfr/Projects/aosp-root/ccache exists
aosp: Checking if /home/jcfr/Projects/aosp-root/ccache exists - ok
docker_entrypoint: Creating user UID/GID [1000/1000]
docker_entrypoint: Creating user UID/GID [1000/1000] - done
docker_entrypoint: Copying .gitconfig and .ssh/config to new user home
docker_entrypoint: Copying .gitconfig and .ssh/config to new user home - done
docker_entrypoint: Creating /tmp/ccache and /asop directory
docker_entrypoint: Creating /tmp/ccache and /asop directory - done
uid=1000(aosp) gid=1000(aosp) groups=1000(aosp)
```
