fix: Docker build and run were not working (#3081)

* fix: Use single quotes instead of backticks when specifying the environment variable that should be substituted for the envsubst command.
GitHub issue: #3079
Removed duplicate calls to envsubst.
Default nginx listen port is now 80.
Adde markdown documentation.

* fix: Fixed some typos and wording in markdown documentation.
GitHub issue: #3079

Co-authored-by: Joe Boccanfuso <joe.boccanfuso@radicalimaging.com>

.docker/Viewer-v3.x/default.conf.template

@@ -1,6 +1,5 @@
 server {
   listen ${PORT};
-  # listen 3000;
   location / {
     root   /usr/share/nginx/html;
     index  index.html index.htm;

.docker/Viewer-v3.x/entrypoint.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-envsubst `${PORT}` < /usr/src/default.conf.template > /etc/nginx/conf.d/default.conf
+envsubst '${PORT}' < /usr/src/default.conf.template > /etc/nginx/conf.d/default.conf
 
 if [ -n "$CLIENT_ID" ] || [ -n "$HEALTHCARE_API_ENDPOINT" ]
   then