Ensure creating proper releases

https://github.com/ethereum/solidity/blob/3b6e131d852eab0c74260d9ca5c7a11501265da1/cmake/scripts/buildinfo.cmake#L22-L26

Author: konradkonrad

Dockerfile

@@ -18,7 +18,11 @@ RUN cd jsoncpp \
 
 WORKDIR /src/solidity/build
 
-RUN git checkout v0.4.2
+ENV RELEASE_TAG v0.4.2
+
+RUN git checkout $RELEASE_TAG
+# https://github.com/ethereum/solidity/blob/3b6e131d852eab0c74260d9ca5c7a11501265da1/cmake/scripts/buildinfo.cmake#L22-L26
+RUN touch ../prerelease.txt
 RUN cmake -DCMAKE_BUILD_TYPE=Release \
           -DTESTS=1 \
           -DSTATIC_LINKING=1 \