add dockerfile and build action for docs

docs/Dockerfile

@@ -0,0 +1,14 @@
+# ---------- STEP 1 ----------
+# Build the documentation in web format
+FROM ghcr.io/vshn/antora:master AS antora
+
+WORKDIR /build
+COPY . /build
+RUN make docs-html antora_cmd=antora antora_opts=--fetch
+
+# ---------- STEP 2 ----------
+# Docker image only containing nginx and the freshly built documentation
+FROM ghcr.io/vshn/nginx:latest
+
+# Finally, copy the contents of the documentation to be served
+COPY --from=antora /build/_public /usr/share/nginx/html