Update Node to v12 in Dockerfile

Signed-off-by: Adrian Nöthlich <git@promasu.tech>

Dockerfile

@@ -1,5 +1,5 @@
 # We use multi stage builds
-FROM node:10-stretch-slim AS build
+FROM node:12-stretch-slim AS build
 
 RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -yq git jq python
 RUN npm install -g bower
@@ -16,7 +16,7 @@ RUN npm install --production \
     && npm install -g bower \
     && bower install --allow-root
 
-FROM node:10-stretch-slim
+FROM node:12-stretch-slim
 
 # You want USE_SSL=true if not putting cryptpad behind a proxy
 ENV USE_SSL=false