Update mega Dockerfile to install Chromium

Per #416 and follow on feedback in #417, adding these deps to the mega
container image to install Chromium and its transient dependencies would
facilitate using Puppeteer in GitHub Actions, per the documentation at
the URL below, deduplicating deps from the dependencies installed in the
base image from the docker/action/Dockerfile base image setup.

https://pptr.dev/troubleshooting/

The i-d-template maintainer and others recommended mermaid-cli as a
viable alternative to aasvg and other tools in #355. The former uses
headless Chromium to operate.

Author: aj-stein-nist

docker/math/Dockerfile

@@ -27,4 +27,11 @@ RUN set -e; \
     rm -rf "$tmp"; \
     apk del .locale_build
 
-RUN apk add npm && npm install -g mathjax-node-cli
+RUN apk add --no-cache \
+    chromium \
+    nss \
+    freetype \
+    harfbuzz \
+    ttf-freefont \
+    yarn && \
+    npm install -g mathjax-node-cli