Allow adding caps to Docker containers

[marmistrz]

I don't like the .clone() in here, because we're not using cap_add later on, but I don't see any clean way to avoid it. (msg is moved into the final closure)

[marmistrz]

The failure on macos is unrelated to this PR:

   Compiling gu-client v0.2.0 (/Users/jenkins/workspace/unlimited-pull-request-macos/gu-client)
error[E0432]: unresolved import `windows_service::service`
  --> gu-provider/src/main.rs:99:26
   |
99 |     use windows_service::service::ServiceType;
   |                          ^^^^^^^ could not find `service` in `windows_service`

error[E0432]: unresolved import `windows_service::service_dispatcher`
   --> gu-provider/src/main.rs:100:9
    |
100 |     use windows_service::service_dispatcher;
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no `service_dispatcher` in the root

error[E0432]: unresolved import `windows_service::service`
   --> gu-provider/src/main.rs:113:30
    |
113 |         use windows_service::service::{
    |                              ^^^^^^^ could not find `service` in `windows_service`

error[E0432]: unresolved imports `windows_service::service_control_handler`, `windows_service::service_control_handler`
   --> gu-provider/src/main.rs:116:56
    |
116 |         use windows_service::service_control_handler::{self, ServiceControlHandlerResult};
    |                              ^^^^^^^^^^^^^^^^^^^^^^^   ^^^^ no `service_control_handler` in the root
    |                              |
    |                              could not find `service_control_handler` in `windows_service`

error: cannot find macro `define_windows_service!` in this scope
   --> gu-provider/src/main.rs:103:5
    |
103 |     define_windows_service!(ffi_service_main, my_service_main);
    |     ^^^^^^^^^^^^^^^^^^^^^^

error[E0412]: cannot find type `Result` in module `windows_service`
   --> gu-provider/src/main.rs:111:42
    |
111 |     fn run_service() -> windows_service::Result<()> {
    |                                          ^^^^^^ not found in `windows_service`
help: possible candidates are found in other modules, you can import them into scope
    |
6   | use actix_web::Result;
    |
6   | use actix_web::error::Result;
    |
6   | use async_docker::Result;
    |
6   | use async_docker::util::Result;
    |
and 21 other candidates

error[E0425]: cannot find value `ffi_service_main` in this scope
   --> gu-provider/src/main.rs:153:53
    |
153 |     let _ = service_dispatcher::start(SERVICE_NAME, ffi_service_main).map_err(|e| {
    |                                                     ^^^^^^^^^^^^^^^^ help: a function with a similar name exists: `my_service_main`

warning: unused `#[macro_use]` import
 --> gu-provider/src/main.rs:2:1
  |
2 | #[macro_use]
  | ^^^^^^^^^^^^
  |
  = note: #[warn(unused_imports)] on by default

warning: use of deprecated item 'actix_web::server::incoming::<impl actix_web::server::http::HttpServer<H, F>>::start_incoming'
   --> gu-provider/src/server.rs:310:40
    |
310 |                         let _ = server.start_incoming(listener.incoming(), false);
    |                                        ^^^^^^^^^^^^^^
    |
    = note: #[warn(deprecated)] on by default

error: aborting due to 7 previous errors