build(docs-infra): unpin certain dependencies of the preview server

Previously, in order to remain as deterministic as possible, the
Dockerfile for the preview server Docker image had all dependencies
pinned to specific versions. It turns out that some packages (such as
`nginx`, `nodejs`, and `openssl` - potentially others too) make older
versions unavailable on the repositories once a newer version is
available.

See for example:
- nodesource/distributions#33
- https://askubuntu.com/questions/715104/how-can-i-downgrade-openssl-via-apt-get

This commit, therefore, removes the exact versions for these packages.
The latest versions will be installed everytime the Docker image is
built (subject to Docker caching).

aio/aio-builds-setup/dockerbuild/Dockerfile

@@ -79,13 +79,18 @@ RUN echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources
 
 
 # Install packages
+# NOTE: Some packages (such as `nginx`, `nodejs`, `openssl`) make older versions unavailable on the
+#       repositories, so we cannot pin to specific versions for these packages :(
+#       See for example:
+#       - https://github.com/nodesource/distributions/issues/33
+#       - https://askubuntu.com/questions/715104/how-can-i-downgrade-openssl-via-apt-get
 RUN apt-get update -y && apt-get install -y \
     cron=3.0pl1-134+deb10u1 \
     dnsmasq=2.80-1 \
     nano=3.2-3 \
-    nginx=1.14.2-2+deb10u1 \
-    nodejs=12.16.2-1nodesource1 \
-    openssl=1.1.1d-0+deb10u3 \
+    nginx \
+    nodejs \
+    openssl \
     rsyslog=8.1901.0-1 \
     vim=2:8.1.0875-5 \
     yarn=1.22.4-1