Add prometheus-alertmanager image.

This patch adds the prometheus-alertmanager[0] component
under the prometheus image namespace.

[0] https://github.com/prometheus/alertmanager

Change-Id: I80c09efceaca18a5e4c0eb4e684be84e565f0cd2
Signed-off-by: Jorge Niedbalski <jorge.niedbalski@linaro.org>

docker/prometheus/prometheus-alertmanager/Dockerfile.j2

@@ -0,0 +1,20 @@
+FROM {{ namespace }}/{{ image_prefix }}prometheus-base:{{ tag }}
+LABEL maintainer="{{ maintainer }}" name="{{ image_name }}" build-date="{{ build_date }}"
+
+{% block prometheus_alertmanager_header %}{% endblock %}
+
+{% block prometheus_alertmanager_repository_version %}
+ENV prometheus_alertmanager_version=0.14.0
+{% endblock %}
+
+{% block prometheus_alertmanager_install %}
+RUN curl -sSL -o /tmp/prometheus_alertmanager.tar.gz https://github.com/prometheus/alertmanager/releases/download/v${prometheus_alertmanager_version}/alertmanager-${prometheus_alertmanager_version}.linux-${prometheus_arch}.tar.gz \
+    && tar xvf /tmp/prometheus_alertmanager.tar.gz -C /opt/ \
+    && rm -f /tmp/prometheus_alertmanager.tar.gz \
+    && ln -s /opt/alertmanager* /opt/prometheus_alertmanager
+{% endblock %}
+
+{% block prometheus_alertmanager_footer %}{% endblock %}
+{% block footer %}{% endblock %}
+
+USER prometheus

releasenotes/notes/prometheus-alertmanager-3dbe1b8ee3b312ee.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - Container image for prometheus alertmanager service under
+    the docker/prometheus/ directory.