consider dockerignore on build step

[selul]

Seems like when the image deployment is used, and if we have a .dockerignore file on the project root, that file is not copied to the .ymir/build directory where the build is happening.

The .dockerignore file is useful to add files that can be ignored on the builded image. It might be similar to #16 but is not related to s3 upload and to the resulting build image.