[YUNIKORN-300] update nginx image (#43)

Security scans have shown known vulnerabilities in the alpine and nginx
version we use to generate the web docker image.
Updating to the latest stable version.

Fixes: #43
diff --git a/Dockerfile b/Dockerfile
index 6a2ea23..96e6004 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -31,7 +31,7 @@
 RUN yarn build:prod
 
 # Stage 2
-FROM nginx:1.15.8-alpine
+FROM nginx:1.18.0-alpine
 
 COPY --from=buildstage /usr/uiapp/dist/yunikorn-web /usr/share/nginx/html
 
diff --git a/nginx/Dockerfile b/nginx/Dockerfile
index 72cfa09..26f1a96 100644
--- a/nginx/Dockerfile
+++ b/nginx/Dockerfile
@@ -1,4 +1,4 @@
-FROM nginx:1.15.8-alpine
+FROM nginx:1.18.0-alpine
 
 COPY ./nginx/nginx.conf /etc/nginx/conf.d/default.conf