Allow configuration of boottime parameters by environment variables

[moriyoshi]

This patch enables configuration of the following boot-time parameters through the environment variables:

• max_connections (POSTGRES_MAX_CONNECTIONS)
• max_prepared_transactions (POSTGRES_MAX_PREPARED_TRANSACTIONS)

[tianon]

Sorry for the long review delay 😞

This can already be accomplished pretty easily without modifying the image, via -c:

$ docker run -dit --name test postgres -c max_connections=200 -c max_prepared_transactions=400 722a07c55a76ec6175cea4d9908e14624f22eef8ee5fdc6bb63a4f5591972f36 $ docker exec -it test psql -U postgres psql (9.6.2) Type "help" for help. postgres=# SHOW max_connections; SHOW max_prepared_transactions;  max_connections  -----------------  200 (1 row)  max_prepared_transactions  ---------------------------  400 (1 row) postgres=# 

Thanks! ❤️