Fix healthcheck when using an ENV file

If someone is using a `.env` file or configured the `ENV_FILE` variable
to use that as it's configuration, this was missed by the healthcheck.

So, `DOMAIN` and `ROCKET_TLS` were not seen, and not used in these cases.

This commit fixes this by checking for this file and if it exists, then
it will load those variables first.

Fixes dani-garcia#4112

docker/healthcheck.sh

@@ -7,6 +7,16 @@
 
 CONFIG_FILE="${DATA_FOLDER}"/config.json
 
+# Check if there is a .env file configured
+# If that is the case, load it into the environment before running any check
+if [ -z "${ENV_FILE}" ]; then
+    ENV_FILE=".env"
+fi
+if [ -r "${ENV_FILE}" ]; then
+    # shellcheck disable=SC1090
+    . "${ENV_FILE}"
+fi
+
 # Given a config key, return the corresponding config value from the
 # config file. If the key doesn't exist, return an empty string.
 get_config_val() {