how to read variable values from application.conf that are in .env file

[mohika-knoldus]

@japgolly

my application.conf file contains variable to be read from .env and when i run this i get an error.Though other values have been read only variables are the problem.
// app.conf
myapp.dbconfig.threads-pool-count = 2
myapp.dbconfig.threads-pool-count = ${?DB_THREAD_POOL_COUNT}
// queueSize The size of the job queue, 0 for direct hand-off or -1 for unlimited size.
myapp.dbconfig.queue-size = 50
myapp.dbconfig.queue-size = ${?DB_QUEUE_SIZE}
myapp.dbconfig.search-limit = 10
myapp.dbconfig.search-limit = ${?SEARCH_LIMIT}

error:-

Exception in thread "main" java.lang.RuntimeException: 3 errors:
[error] - Error reading key [myapp.dbconfig.queue-size] from source [cp:/application.conf] with value [${DB_QUEUE_SIZE}]: Int expected.
[error] - Error reading key [myapp.dbconfig.search-limit] from source [cp:/application.conf] with value [${SEARCH_LIMIT}]: Int expected.
[error] - Error reading key [myapp.dbconfig.threads-pool-count] from source [cp:/application.conf] with value [${DB_THREAD_POOL_COUNT}]: Int expected.
[error] 5 sources (highest to lowest priority):
[error] - Env