Add Dockerfile without pre-vendoring

[ilyaglow]

Hi there,

I added a novendor.Dockerfile, that can be used without doing go mod vendor first.

It allows users to build benthos-lab directly from the repository without even cloning it, like this:

docker build -t benthos-lab https://github.com/ilyaglow/benthos-lab.git#novendor-dockerfile -f novendor.Dockerfile

Also, I reduced the number of RUN command layers and removed the WORKDIR command in the package stage, which is unnecessary since it's a default working directory.

[Jeffail]

Thanks @ilyaglow!