Bump node to 15.14 + add .nvmrc

.nvmrc

@@ -0,0 +1 @@
+15.14

README.md

@@ -28,7 +28,9 @@ make sure to properly set up and secure your server yourself.
 1. Create a user for running the application. It is not recommended to run as a
    user that is also used for other things. Do not run as root!
 2. Clone the repository.
-3. Install `node` of at least version 14, and also `npm`.
+3. Install `node` of at least version 15, and also `npm`. You can use the 
+   [Node Version Manager (npm)](https://github.com/nvm-sh/nvm), since this project
+   contains a `.nvmrc`. Just run `nvm install` followed by `nvm use`.
 4. Run `npm ci` in the `api` and `frontend` directories to install dependencies.
 5. Install and configure a MongoDB somewhere.
 6. Copy `api/config.json.example` to `api/config.json` and change it to suit

api/Dockerfile

@@ -1,4 +1,4 @@
-FROM node:15.11.0-buster
+FROM node:15.14-buster
 
 # Install python3, pip3, and make them the default for `python` and `pip` commands
 RUN apt-get update && apt-get install -y python3 python3-pip

frontend/Dockerfile

@@ -1,4 +1,4 @@
-FROM node:14
+FROM node:15.14-buster
 
 WORKDIR /opt/obs/frontend
 ADD package.json package-lock.json /opt/obs/frontend/