add healthcheck for http services

core/nginx/Dockerfile

@@ -1,6 +1,6 @@
 FROM alpine:3.8
 
-RUN apk add --no-cache certbot nginx nginx-mod-mail openssl \
+RUN apk add --no-cache certbot nginx nginx-mod-mail openssl curl \
  python py-jinja2 py-requests-toolbelt py-pip \
  && pip install --upgrade pip \
  && pip install idna
@@ -12,3 +12,5 @@ EXPOSE 80/tcp 443/tcp 110/tcp 143/tcp 465/tcp 587/tcp 993/tcp 995/tcp 25/tcp 100
 VOLUME ["/certs"]
 
 CMD /start.py
+
+HEALTHCHECK CMD curl -f -L http://localhost/ || exit 1