Waste fewer layers when copying resources

Before this PR, the `docker-maven-plugin` would create one layer per copied file in a `<resources>` block. This now collates files that end up in the same directory into one command. This also uses `COPY` instead of `ADD` for cleaner semantics.

**tl;dr** before:

    COPY a.txt /foo/a.txt
    COPY b.txt /foo/b.txt
    COPY c.txt /foo/a.txt

after:

    COPY a.txt b.txt c.txt /foo/a.txt