docker: install GNU sed to fix ansi2html (fixes chubin#274, chubin#301)

Author: abitrolly

Dockerfile

@@ -2,7 +2,7 @@ FROM alpine:3.14
 # fetching cheat sheets
 ## installing dependencies
 RUN apk add --update --no-cache git py3-six py3-pygments py3-yaml py3-gevent \
-      libstdc++ py3-colorama py3-requests py3-icu py3-redis
+      libstdc++ py3-colorama py3-requests py3-icu py3-redis sed
 ## copying
 WORKDIR /app
 COPY . /app