Pass cargo configuration flags to docker.

Ensures the following flags are passed to docker:
- RUSTDOCFLAGS
- RUSTFLAGS
- CARGO_INCREMENTAL
- CARGO_CACHE_RUSTC_INFO
- HTTPS_PROXY
- HTTP_TIMEOUT
- TERM
- BROWSER

These are always correct, since these configure how `cargo` behavior is
supposed to work, but don't rely on any paths on the host filesystem.

src/docker/shared.rs

@@ -249,6 +249,17 @@ pub(crate) fn docker_envvars(docker: &mut Command, config: &Config, target: &Tar
         .args(&["-e", "CARGO_TARGET_DIR=/target"])
         .args(&["-e", &cross_runner]);
 
+    // also need to accept any additional flags passed to cargo
+    docker
+        .args(&["-e", "RUSTDOCFLAGS"])
+        .args(&["-e", "RUSTFLAGS"])
+        .args(&["-e", "CARGO_INCREMENTAL"])
+        .args(&["-e", "CARGO_CACHE_RUSTC_INFO"])
+        .args(&["-e", "HTTPS_PROXY"])
+        .args(&["-e", "HTTP_TIMEOUT"])
+        .args(&["-e", "TERM"])
+        .args(&["-e", "BROWSER"]);
+
     if let Some(username) = id::username().unwrap() {
         docker.args(&["-e", &format!("USER={username}")]);
     }