diff --git a/crates/sandcage/src/docker.rs b/crates/sandcage/src/docker.rs
index 9525e48..d011d9b 100644
--- a/crates/sandcage/src/docker.rs
+++ b/crates/sandcage/src/docker.rs
@@ -879,6 +879,11 @@ async fn run_docker_build(
     let mut cmd = Command::new(docker);
     cmd.args(["build", "-t", &format!("{image}:latest")]);
 
+    let uid = id_flag("-u").await.unwrap_or_else(|_| "1000".into());
+    let gid = id_flag("-g").await.unwrap_or_else(|_| "1000".into());
+    cmd.args(["--build-arg", &format!("SANDCAGE_UID={uid}")]);
+    cmd.args(["--build-arg", &format!("SANDCAGE_GID={gid}")]);
+
     if no_cache {
         cmd.arg("--no-cache");
     }
@@ -1000,15 +1005,20 @@ pub async fn build_images(force: bool, service_filter: &[String]) -> Result<()>
     let hashes_path = build_hashes_path()?;
     let mut stored = read_stored_hashes(&hashes_path)?;
 
+    let uid = id_flag("-u").await.unwrap_or_else(|_| "1000".into());
+    let gid = id_flag("-g").await.unwrap_or_else(|_| "1000".into());
+    let uid_suffix = format!("\n# uid={uid} gid={gid}");
+
     for (image, bundled_dockerfile) in images {
         let short_name = image.strip_prefix("sandcage-").unwrap_or(image);
         let override_path = overrides.get(short_name);
 
-        let current_hash = if let Some(path) = override_path {
-            sha256_hex(&read_dockerfile_at(path)?)
+        let dockerfile_content = if let Some(path) = override_path {
+            read_dockerfile_at(path)?
         } else {
-            sha256_hex(bundled_dockerfile)
+            bundled_dockerfile.to_string()
         };
+        let current_hash = sha256_hex(&format!("{dockerfile_content}{uid_suffix}"));
 
         let needs_build = force || stored.get(image) != Some(&current_hash);