The problem has nothing to do with git but everything to do with the code. Apparently, this commit is bad:
https://github.com/zookzook/elixir-mongodb-driver/commit/e00871509c5287cd212d4c6f6c1783a0a64692b6

in mix.exs, project/0 calls docs/0, which in turn calls project/0 again, it is an infinite recursion. Did they test their code before the push?

It’s very likely your SSH software is waiting for you to confirm you want to add an IP to list of trusted hosts (in the spawned shell when fetching dependencies).

Happened to me many times in Docker deployments.

Suggestion: don’t use the GIT URL that’s usually used for push access. Use the https:// URL variant.

Bonus points: just use the github: "user/library" syntax in mix.exs and forget about this nonsense.

I’m becoming more and more convinced that this is something to do with the commit that’s on the head of the master branch on the mongodb_driver repo. Is it a merge commit? Why should that be problematic?

Using the HTTP scheme for the URL had no effect. The following still hangs:

# Fails. (mix hangs)
{:mongodb_driver, git: "https://github.com/zookzook/elixir-mongodb-driver.git", branch: "master"}

Using the github syntax fails:

# Fails. (mix hangs)
{:mongodb_driver, github: "zookzook/elixir-mongodb-driver", branch: "master"}

Avoiding the problematic commit by specifying a ref works with the HTTP URL scheme:

# Ok!
{:mongodb_driver, git: "https://github.com/zookzook/elixir-mongodb-driver.git", ref: "d7c8652b4e66a0031e1a7fe36bc2af5c85cef408"}

Likewise, it works with the github syntax:

# Ok!
{:mongodb_driver, github: "zookzook/elixir-mongodb-driver", ref: "d7c8652b4e66a0031e1a7fe36bc2af5c85cef408"}

So something is weird with the 59aa841cc619f77979cc3027c76e918373685723 that’s at the HEAD of the master branch. Somebody with more git foo than me will have to clarify what the problem is. It’s still surprising that this one thing is preventing ALL mix tasks from running.