tools/ci/docker/linux/Dockerfile: fix ccache when running container as user

[userid0x0]

Normally ccache places the temporary files in ~/.ccache, when invoking the container with
-u <uid>:<gid>
as e.g. Jenkins does, this might result in a invalid/unknown home folder. Thus ccache tries to write to /.ccache which doesn't exist and the user doesn't have any permissions.

Explicitly set CCACHE_DIR to a folder with appropriate permissions.

Summary

Impact

Testing

Before:

# docker run --rm --tty --interactive -u 1000:1000 ghcr.io/apache/nuttx/apache-nuttx-ci-linux:latest
cd /tmp
touch test.c
arm-none-eabi-gcc -c test.c
# ccache: error: Failed to create directory //.cache/ccache/tmp: Permission denied