docker: few nits

Author: csegarragonz

docker/build.dockerfile

@@ -2,7 +2,7 @@ ARG CPP_VERSION
 ARG EXAMPLES_VERSION
 # Base image is not re-built often and tag may lag behind
 FROM faasm.azurecr.io/examples-base:0.6.0_0.4.0 AS base
-FROM faasm.azurecr.io/cpp-sysroot:${CPP_VERSION}
+FROM faasm.azurecr.io/cpp-sysroot:${CPP_VERSION:-dead}
 
 SHELL ["/bin/bash", "-c"]
 ENV IN_DOCKER="on"
@@ -50,6 +50,7 @@ RUN mkdir -p code \
     && git submodule update --init -f examples/tensorflow
 
 # Build the examples and demo functions
+ENV PATH=${PATH}:/root/.cargo/bin
 RUN cd /code/examples \
     && ./bin/create_venv.sh \
     && source venv/bin/activate \