Allow setting healthcheck domain with an environment variable

[tnyeanderson]

It should be easy to set the domain for the HEALTHCHECK without having to copy/repeat/redefine the entire health check command.

This PR:

• Adds an environment variable HEALTHCHECK_DOMAIN to the container with default value cloudflare.com
• Replaces cloudflare.com in the health check command with "${HEALTHCHECK_DOMAIN}"

Therefore allowing the user to set which domain is queried for the health check.

Due to the directory structure, a diff is provided below for clarity:

diff -urN 1.16.2/Dockerfile 1.16.3/Dockerfile
--- 1.16.2/Dockerfile   2022-09-07 22:31:39.284900810 -0400
+++ 1.16.3/Dockerfile   2022-09-07 22:38:06.670352779 -0400
@@ -102,6 +102,8 @@
     SUMMARY="${NAME} is a validating, recursive, and caching DNS resolver." \
     DESCRIPTION="${NAME} is a validating, recursive, and caching DNS resolver."

+ENV HEALTHCHECK_DOMAIN=cloudflare.com
+
 WORKDIR /tmp/src

 COPY --from=unbound /opt /opt
@@ -144,6 +146,6 @@
 EXPOSE 53/tcp
 EXPOSE 53/udp

-HEALTHCHECK --interval=30s --timeout=30s --start-period=10s --retries=3 CMD drill @127.0.0.1 cloudflare.com || exit 1
+HEALTHCHECK --interval=30s --timeout=30s --start-period=10s --retries=3 CMD drill @127.0.0.1 "${HEALTHCHECK_DOMAIN}" || exit 1

 CMD ["/unbound.sh"]

Retrieved with:

diff -urN 1.16.{2,3}