Dockerfile: avoid usage errors

Dockerfile

@@ -1,9 +1,18 @@
+
+# this Dockerfile is expected to run with a checkout
+# of the vzlogger source as the build context (working directory),
+# which it will copy into the container and then build.
+
 ############################
 # STEP 1 build executable binary
 ############################
 
 FROM alpine:latest as builder
 
+COPY . /vzlogger
+# ensure we error early if there is no source in the build context
+COPY CMakeLists.txt /vzlogger/
+
 RUN apk add --no-cache \
     gcc \
     g++ \
@@ -37,8 +46,6 @@ RUN git clone https://github.com/rscada/libmbus.git --depth 1 \
     && ./build.sh \
     && make install
 
-COPY . /vzlogger
-
 ARG build_test=off
 RUN cmake -DBUILD_TEST=${build_test} \
     && make -j $(nproc --all || echo 1) \