fix: Expand MongoDB credentials

[achetronic]

Hello there :)

When using external MongoDB, this docker image allow defining a mongodb connection string using the variable EAP_MONGOD_URI with the credentials inside as follows mongodb://root:whatever@your-ip-here/omada. This is working fine.

But in Kubernetes, the environment variables are usually loaded from a ConfigMap, and this type of manifests are commonly stored on git. However credentials are usually stored on a Secret resource, provisioned dynamically by a different flow.

As this image is not having different environment variables to define MongoDB user or password, we need a way to get them from another environment variables and put the content into the string. Some examples:

mongodb://root:$MONGODB_ROOT_PASSWORD@$MONGO_HOST/omada
mongodb://root:${MONGODB_ROOT_PASSWORD}@${MONGO_HOST}/omada

With the change I propose, this is covered as the content of the variable is expanded, so if the environment variable is defined into the connection string, it will be replaced with its content

[mbentley]

Nice, I like it - a simple solution too. Thanks for the PR. LGTM.