Edit: Ironically Docker itself has the potential to help solve the problem of running untrusted open source code. I think every open source project should include either a Dockerfile or Vagrantfile to help users get up and running quickly, and safely run untrusted projects.
For example, I would notice that it requires apt-get
echo "Ensuring basic dependencies are installed..."
apt-get -qq update
apt-get -qq install lxc wget bsdtar
I don't know much about docker, but doing a "curl | sh" peeks my interest, then downloading additional binaries into /usr/local/bin, I'd want to take a closer look. Obviously, this is a case by case review, till it sits well with me. If I was going to run this in production, I'd want to have a really good idea of what this was doing and what to expect, so I'd probably take a closer look at the source if it was not clear from the documentation.
That is all.
This can go on pretty far - trust is always an eventually unsolvable issue.
There is a certain level of trust that is easy to achieve and easy to get. Trusting dotcloud is easier than trusting everyone on the internet is pink bunny. Happens to be that HTTPS and signing aren't exactly hard either ;-)
You either have to trust Docker (a fairly well known project built by reputable people) isn't going to root your machine, or download the source yourself and audit it.
This is no worse than suggesting you "git clone whatever; cd whatever; make" (aside from the lack of SSL)
...and everybody else on my network, with that method. Doing that I don't even get the chance to think "Hey wait a second, why was this only 50 bytes of shell script...".
The reason that you see outrage for this "method" is because it is born of laziness and far too reminiscent of more disturbing times in computer security.
Why do we need an instruction on downloading the source to begin with? It really just promotes bad habits with those who know no better, i.e. new/inexperienced developers. The problem is when people see instructions like that on 20% of the guides they read in earnest, trusting that everything is OK if enough people say it. One hopes they stumble upon a discussion like this so that they can consider the consequences but that just isn't going to happen to everyone. True, one should exercise equal caution while cloning, gem-ing[1], etc. It would be great if authors would just link to the source and paste the relevant lines from the README if necessary.
[1] http://andre.arko.net/2013/03/29/rubygems-openssl-and-you/
Ideally it would download a file from Github too, that way you can be sure it's coming straight from the publicly visible open source repo, and you can audit if you want.
But I think the general outrage over this technique is overblown.