Update README.md (#3039)

Add lines to discuss silent DB failures related to background postgres processes.

containers/ecr-viewer/README.md

@@ -50,6 +50,8 @@ If you have problems connecting to your database, use this command to see what o
 
 If building the Docker Image doesn't work as expected, try to first run the eCR Viewer locally using the steps below.
 
+If you consistently encounter the error message `"ecr_viewer_db" does not exist` when attempting to run the app, there could be conflicting databases running on port 5432 as part of other background processes. Try pruning any dangling Docker images and containers (`docker image prune` and `docker container prune`). If issues persist, try logging into `psql` on the command line to see what databases are running there. 
+
 ## Development
 
 ### Run eCR Viewer Locally