diff --git a/src/docker_util.rs b/src/docker_util.rs
index 362f9c4..a8bd684 100644
--- a/src/docker_util.rs
+++ b/src/docker_util.rs
@@ -81,7 +81,7 @@ impl DockerCmd {
             },
         };
 
-        let mut sub_cmd = String::new();
+        let mut sub_cmd = String::with_capacity(1024);
         if let Some(mirror) = self.mirror {
             sub_cmd.push_str(&make_cmd(&mirror));
             sub_cmd.push_str("\n");
@@ -118,7 +118,7 @@ impl DockerCmd {
 }
 
 fn escape_arg(arg: &str) -> String {
-    let mut r = String::new();
+    let mut r = String::with_capacity(arg.len() + 10);
     r.push('\'');
     for c in arg.chars() {
         if c == '\'' {
@@ -131,7 +131,7 @@ fn escape_arg(arg: &str) -> String {
 }
 
 fn make_cmd(mirror: &str) -> String {
-    let mut s = String::new();
+    let mut s = String::with_capacity(256);
     s.push_str(&format!(r#"echo '[source.crates-io]
     registry = "https://github.com/rust-lang/crates.io-index"
     replace-with = "mirror"