Fix librdkafka in final Docker image (#36)

* Bump librdkafka in final image, add test

Fixes a bug where prometheus-kafka-adapter wouldn't start because
librdkafka wasn't bumped in the final image of the multi-stage build.
Also adds a test to Travis that verifies the image can be started.

* Add stages to Travis build

.travis.yml

@@ -6,10 +6,17 @@ language: minimal
 
 services:
   - docker
+jobs:
+  include:
+    - stage: install
+      script: docker build -t telefonica/prometheus-kafka-adapter .
 
-install:
-  - docker build -t telefonica/prometheus-kafka-adapter .
+    - stage: test
+      script:
+        - docker build -t telefonica/prometheus-kafka-adapter .
+        - docker run -d --name test telefonica/prometheus-kafka-adapter
+        - if [ $(docker inspect -f {{.State.Status}} test) = "running" ]; then echo "OK"; exit 0; else echo "FAILED"; exit 1; fi
 
 git:
   depth: 1
-  quiet: true
+  quiet: true

Dockerfile

@@ -11,7 +11,9 @@ RUN go build -o /prometheus-kafka-adapter
 
 FROM alpine:3.11
 
-RUN apk add --no-cache librdkafka
+RUN echo "@edge http://dl-cdn.alpinelinux.org/alpine/edge/main" >> /etc/apk/repositories && \
+    echo "@edgecommunity http://dl-cdn.alpinelinux.org/alpine/edge/community" >> /etc/apk/repositories && \
+    apk add --no-cache 'librdkafka@edgecommunity>=1.3.0'
 
 COPY --from=build /src/prometheus-kafka-adapter/schemas/metric.avsc /schemas/metric.avsc
 COPY --from=build /prometheus-kafka-adapter /