`init` replaces Dockerfile if it already exists

[dszlachta]

If the project already has Dockerfile in the main directory, running dappnodesdk init will replace it without any warning. This can be a serious problem if the user is just quickly prototyping and they haven't call git init yet.

To Reproduce

1. Install version 0.2.69
2. In the terminal, do

cd /tmp
mkdir test && cd test
echo "Content" > Dockerfile # create the file
npx @dappnode/dappnodesdk init
cat Dockerfile # note the content is different

Expected behavior

Because init creates boilerplate Dockerfile, I guess it would be OK to simply skip this step if Dockerfile already exists

DAppNode version:

I don't think it matters here

Additional context

As always, DAppNode is awesome. Thanks.