Dockerfile: Bump Rust to version 1.56 to fix build error

Error was:

  error[E0658]: use of unstable library feature 'str_strip': newly added
    --> /usr/local/cargo/registry/src/github.com-1ecc6299db9ec823/impl-serde-0.3.2/src/serialize.rs:97:24
     |
  97 |     let (v, stripped) = v.strip_prefix("0x").map_or((v, false), |v| (v, true));
     |                           ^^^^^^^^^^^^
     |
     = note: see issue #67302 <rust-lang/rust#67302> for more information

Dockerfile

@@ -1,4 +1,4 @@
-FROM rust:1.44 as build
+FROM rust:1.56 as build
 
 ENV BASE /opt/cartesi
 RUN \