Support ssh git dependencies in pip requirements.txt when building with use-container

### Describe your idea/feature/enhancement

In a requirements.txt file I can have something like this:

git+ssh://git@bitbucket.org/path/to/my.git

If I do a sam build on this, it works. But if I add --use-container it does not.

Host key verification failed.
fatal: Could not read from remote repository.

I presume this is a combination of my SSH key plus known_hosts not being taken from the host.

### Proposal

The only 2 solutions I can think of are either to copy the keys onto the container (hmm) or to clone them on the host, copy them onto the container then run pip install on them.

Neither seems ideal to me (and I can see why you wouldn't want to do the first one), but I'm hoping that there is maybe something I've not thought of.

I'd like to continue to use --use-container but we also need a way to share code between lambdas and this seems to me to be the easiest. I don't know much about hosting a private pypi yet but I presume I'll run into a similar problem there.