Build: Enable the ability to build install/preview:docker in Werft again

[mads-hartmann]

In #13093 install/preview:docker was turned into a dynamic package to make it optional to build. However, it appears that dynamic packages currently break leeway exec (see gitpod-io/leeway#100) and due to a misunderstanding install/preview:docker was removed from the "all" package dependency tree in #13240. That means that currently, you can't build install/preview:docker in CI.

The easiest fix is that doesn't break leeway exec is the

1. Convert it back to a "normal" package
2. But keep it out of the ":all" package dependency tree
3. Instead use leeway build install/preview:docker in Werft when the --with-local-preview is enabled.

⬆ This also means that it's easier to build the package in Gitpod as you don't have to pass in the build argument that the dynamic package relied on.