docker workaround MSYS path conversion issue

Without this on a Windows system with Git for Windows installed, I get:
```
docker: Error response from daemon: the working directory '\workspace\' is invalid, it needs to be an absolute path.
```

Apparently this is a known issue with Git for Windows: docker/for-win#1971 (comment)

It seems harmless to prepend a slash on any platform, to work around this.

src/DockerRunner.jl

@@ -78,7 +78,7 @@ function DockerRunner(workspace_root::String;
     docker_cmd = `docker run --privileged `#--cap-add SYS_ADMIN`
 
     if cwd != nothing
-        docker_cmd = `$docker_cmd -w $(abspath(cwd))`
+        docker_cmd = `$docker_cmd -w /$(abspath(cwd))`
     end
 
     # Add in read-only mappings and read-write workspaces