gitignore: Add docker build assets path

Currently after building the docker images locally the git starts showing
the generated files as untracked. One needs to be careful and not use
`git add .` to avoid adding them accidentally.

Just add these generated files in docker/assets path in .gitignore to
avoid such accidental addition.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>