Docker: Update to the latest LTS image

The Node.js 14 slim image has quite a few vulnerabilities, and I have
tested the latest slim image. It works just fine.

When installing plugins, `--legacy-peer-deps` is passed to npm because
npm v7 (which comes with Node.js v16, the current LTS) changed how
peer deps are handled. The new behavior is incompatible with how
plugins have historically been installed.

Dockerfile

@@ -4,7 +4,7 @@
 #
 # Author: muxator
 
-FROM node:14-buster-slim
+FROM node:lts-slim
 LABEL maintainer="Etherpad team, https://github.com/ether/etherpad-lite"
 
 # plugins to install while building the container. By default no plugins are
@@ -85,7 +85,7 @@ COPY --chown=etherpad:etherpad ./ ./
 # seems to confuse tools such as `npm outdated`, `npm update`, and some ESLint
 # rules.
 RUN { [ -z "${ETHERPAD_PLUGINS}" ] || \
-      npm install --no-save ${ETHERPAD_PLUGINS}; } && \
+      npm install --no-save --legacy-peer-deps ${ETHERPAD_PLUGINS}; } && \
     src/bin/installDeps.sh && \
     rm -rf ~/.npm