Fix compilation with newer rust-musl version (#7453)

docker/Dockerfile.j2

@@ -106,7 +106,7 @@ WORKDIR /app
 
 {% if base == "alpine" %}
 # Environment variables for Cargo on Alpine based builds
-RUN echo "export CARGO_TARGET=${RUST_MUSL_CROSS_TARGET}" >> /env-cargo && \
+RUN echo "export CARGO_TARGET=${CARGO_BUILD_TARGET}" >> /env-cargo && \
     # Output the current contents of the file
     cat /env-cargo