Fix missing Dockerfile ARG, Update Rust to 1.49.0 (#67)

* Fix missing Dockerfile ARG

Docker ARG REPOS_FILE was not redeclared in the "cacher" build stage,
and so was not inheriting the default value specified at the top of the
file. This redeclares the arg in the "cacher" build stage, so that the
default value may be used.

* Update Rust in Dockerfile to 1.49.0

rclrs_examples fail to compile due to the following error
```
error[E0658]: use of unstable library feature 'renamed_spin_loop'
  --> /usr/local/cargo/registry/src/github.com-1ecc6299db9ec823/spin-0.9.2/src/relax.rs:26:9
   |
26 |         core::hint::spin_loop();
   |         ^^^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #55002 <rust-lang/rust#55002> for more information

   Compiling bitvec v0.19.5
error: aborting due to previous error
```

As seen in the noted link, updating Rust can resolve the issue. This
updates Rust in the Dockerfile to 1.49.0, where the issue seems to be
resolved.

Dockerfile

@@ -21,6 +21,7 @@ FROM $FROM_IMAGE AS cacher
 
 # clone overlay source
 ARG OVERLAY_WS
+ARG REPOS_FILE
 WORKDIR $OVERLAY_WS/src
 COPY ./${REPOS_FILE} ../
 RUN vcs import ./ < ../${REPOS_FILE} && \
@@ -53,7 +54,7 @@ RUN apt-get update && apt-get install -q -y \
 ENV RUSTUP_HOME=/usr/local/rustup \
     CARGO_HOME=/usr/local/cargo \
     PATH=/usr/local/cargo/bin:$PATH \
-    RUST_VERSION=1.47.0
+    RUST_VERSION=1.49.0
 RUN set -eux; \
     wget -O rustup-init "https://sh.rustup.rs"; \
     chmod +x rustup-init; \