Enable concurrent CI builds (#1236)

Currently, only one CI build can be started at the same time due to we only have only one testing cluster. This PR aims to enable multiple concurrent builds can be triggered to reduce the waiting time for CI builds. The major changes listed below:

1. Each build generate unique images for testing and debugging
2. Check whether the testing cluster is being used by another build.
3. Cache docker image & go mod after build finished
4. Improve dockerfiles for reusing docker build cache

environments/python/Dockerfile-2.7

@@ -1,14 +1,15 @@
 FROM alpine:3.5
 
-RUN apk update
-RUN apk add --no-cache python python-dev build-base py-pip libev-dev
-RUN pip install --upgrade pip
-RUN rm -r /root/.cache
+WORKDIR /app
+
+RUN apk add --no-cache python python-dev build-base py-pip libev-dev && \
+    pip install --upgrade pip && \
+    rm -r /root/.cache
+
+COPY requirements.txt /app
+RUN pip install -r requirements.txt
 
 COPY . /app
-WORKDIR /app
-RUN pip install -r requirements.txt
 
 ENTRYPOINT ["python"]
 CMD ["server.py"]
-