Fix Travis building of "management" variants

Author: tianon

.travis.yml

@@ -11,7 +11,7 @@ install:
 before_script:
   - env | sort
   - cd "$VERSION"
-  - image="rabbitmq:$VERSION-$VARIANT"
+  - image="$(awk 'toupper($1) == "FROM" { print $2; exit }' management/Dockerfile)"
 
 script:
   - travis_retry docker build -t "$image" "$VARIANT"

3.6/alpine/management/Dockerfile

@@ -1,4 +1,4 @@
-FROM rabbitmq:alpine
+FROM rabbitmq:3.6-alpine
 
 RUN rabbitmq-plugins enable --offline rabbitmq_management
 

3.6/debian/management/Dockerfile

@@ -1,4 +1,4 @@
-FROM rabbitmq
+FROM rabbitmq:3.6
 
 RUN rabbitmq-plugins enable --offline rabbitmq_management