Added nsswitch so that /etc/hosts file will work.

roles/prom_server/tasks/main.yml

@@ -30,8 +30,13 @@
     - alerting.rules
     - targets.json
 
-  tags:
-    - service-files
+- name: Install nsswitch see https://github.com/jumanjiman/docker-ssllabs-scan/blob/6d9f48c9ac4f1df3eebef3ab28e2dd44a9ba4998/scanner/etc/nsswitch.conf
+  copy:
+    src: templates/etc/nsswitch.conf
+    dest: /srv/prometheus/etc/nsswitch.conf
+    mode: 644
+    owner: root
+    group: root
 
 - name: Install service files.
   template:

roles/prom_server/templates/etc/nsswitch.conf

@@ -0,0 +1,3 @@
+# https://github.com/golang/go/blob/go1.9.1/src/net/conf.go#L194-L275
+# https://golang.org/pkg/net/
+hosts: files dns

roles/prom_server/templates/prometheus.service

@@ -11,6 +11,7 @@ ExecStartPre=-/usr/bin/docker rm %n
 ExecStart=/usr/bin/docker run --name %n \
     --network host \
     -v /srv/prometheus/prometheus:/prometheus  \
+    -v /srv/prometheus/etc/nsswitch.conf/etc/nsswitch.conf  \
     -v /srv/prometheus/etc/prometheus:/etc/prometheus  \
     prom/prometheus:v2.6.0 \
     --storage.tsdb.retention 365d --config.file=/etc/prometheus/prometheus.yml \