Create Docker container to host the xblock-sdk

This implements an initial Docker container to host an instance of the
OpenEdX XBlock SDK which is to be used during XBlock development.
These variables probably _shouldn't_ exist in the ENV (as named) and I
wanted to avoid going down the overly verbose path of
```shell
make XBLOCK_SDK_TEMPLATE=... XBLOCK_SDK_REPO=...
```

and instead preferred keeping it simple like
```shell
make TEMPLATE=... SDK=...
```