change cp with tar

[auk]

tar is more proper tool for files cloning - it preserves owners, permissions, works better with tons of small files

[felipecrs]

The cp -a also preserves these attributes.

[datocrats-org]

added justification: docker engine uses tar https://docs.docker.com/engine/reference/commandline/export/ and tar is faster than cp https://superuser.com/a/985080

[harmv]

take care. cp -a of busybox (which is in alpine base image) does not preserve hard links.