Update docker to 3.0.1

.circleci/config.yml

@@ -3,7 +3,7 @@ version: 2
 jobs:
   build:
     docker:
-      - image: nemtrif/utf8cpp:3.0
+      - image: nemtrif/utf8cpp:3.0.1
     steps:
       - checkout
       - run: git submodule update --init --recursive --remote

tests/docker/Dockerfile

@@ -1,5 +1,5 @@
 FROM debian:stretch-slim
 
 RUN  apt-get update \
-  && apt-get install -y make g++ cmake \
+  && apt-get install -y make g++ cmake git \
   && rm -rf /var/lib/apt/lists/*