ci: set env CI to true (#418)

Dockerfile

@@ -1,5 +1,4 @@
 ARG NODE_VERSION
-
 FROM node:${NODE_VERSION}-buster-slim
 
 SHELL ["/bin/bash", "-l", "-c"]
@@ -26,6 +25,8 @@ RUN wget -q "https://github.com/git/git/archive/v${GIT_VERSION}.tar.gz" && \
     make all && \
     make install
 
+ENV CI="true"
+
 WORKDIR /repo
 
 CMD [ "bash" ]