Initial commit with basic support for remote docker.

This supports the volume-based structure, and uses some nice
optimizations to ensure that only the desired toolchain and cargo items
are copied over. It also uses drops to ensure scoped deletion of
resources, to avoid complex logic ensuring their cleanup.

It also supports persistent data volumes, through `cross-util`. In order to setup a persistent data volume, use:

```bash
cross-util create-crate-volume --target arm-unknown-linux-gnueabihf
```

Make sure you provide your `DOCKER_HOST` or correct engine type to ensure these are being made on the remote host. Then, run your command as before:

```bash
CROSS_REMOTE=true cross build --target arm-unknown-linux-gnueabihf
```

Finally, you can clean up the generated volume using:

```bash
cross-util remove-crate-volume --target arm-unknown-linux-gnueabihf
```

A few other utilities are present in `cross-util`:
- `list-volumes`: list all volumes created by cross.
- `remove-volumes`: remove all volumes created by cross.
- `prune-volumes`: prune all volumes unassociated with a container.
- `list-containers`: list all active containers created by cross.
- `remove-containers`: remove all active containers created by cross.

The initial implementation was done by Marc Schreiber, https://github.com/schrieveslaach.

A few more environment variables exist to fine-tune performance, as well as handle private dependencies.
- `CROSS_REMOTE_COPY_REGISTRY`: copy the cargo registry
- `CROSS_REMOTE_COPY_CACHE`: copy cache directories, including the target directory.

Fixes cross-rs#248.
Fixes cross-rs#273.
Closes cross-rs#449.

CHANGELOG.md

@@ -7,6 +7,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Added
 
+- #785 - added support for remote container engines through data volumes. also adds in utility to commands to create and remove persistent data volumes.
 - #782 - added `build-std` config option, which builds the rust standard library from source if enabled.
 - #775 - forward Cargo exit code to host
 - #772 - added `CROSS_CONTAINER_OPTS` environment variable to replace `DOCKER_OPTS`.

Cargo.toml

@@ -31,6 +31,9 @@ serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 serde_ignored = "0.1.2"
 shell-words = "1.1.0"
+walkdir = { version = "2", optional = true }
+sha1_smol = "1.0.0"
+tempfile = "3.3.0"
 
 [target.'cfg(not(windows))'.dependencies]
 nix = { version = "0.24", default-features = false, features = ["user"] }