Gogs cloning with ssh and Docker

Just wondering how one might go about retrieving libraries with an ssh key using the Docker container. We use exclusively ssh for getting our packages using Gogs. So our repository definitions look like:

```
"repositories": [ { "type": "git", "url": "git@git.xxx.com:xxx/xxx.git" } ],
```
and then on our systems we have `~/.ssh/config`s to permit access over that url.

Right now when I try to grab packages, I'm getting
```
[RuntimeException]
Failed to execute git clone --mirror 'git@git.xxx.com:xxx/xxx.git' '/composer/cache/vcs/git-git.xxx.com-xxx.git/'
Cloning into bare repository '/composer/cache/vcs/git-git.xxx.xxx.git'...
Warning: Permanently added 'xxx' (ECDSA)
to the list of known hosts.
Permission denied (publickey).
fatal: Could not read from remote repository.
Please make sure you have the correct access rights
and the repository exists.
```

, which is understandable because I haven't configured any key for accessing the git server.

No source file or test is named. Start by reproducing the Composer git clone inside the Docker container and review how the container receives SSH keys and known-host configuration. Done means the documented setup allows the shown Gogs SSH repository to clone successfully.

Written by the indexing model from the issue text.