Unexpected PostgreSQL restart

My PostgreSQL server unexpectedly restarted with such messages:

2017-08-16 03:44:34 GMT LOG:  received fast shutdown request
2017-08-16 03:44:34 GMT LOG:  aborting any active transactions
2017-08-16 03:44:34 GMT FATAL:  terminating connection due to administrator command
2017-08-16 03:44:34 GMT FATAL:  terminating connection due to administrator command
2017-08-16 03:44:34 GMT LOG:  autovacuum launcher shutting down
2017-08-16 03:44:34 GMT LOG:  shutting down
2017-08-16 03:44:34 GMT LOG:  database system is shut down
2017-08-16 03:46:04 GMT LOG:  incomplete startup packet
2017-08-16 03:46:04 GMT LOG:  database system was shut down at 2017-08-16 03:44:34 GMT
2017-08-16 03:46:04 GMT LOG:  MultiXact member wraparound protections are now enabled
2017-08-16 03:46:04 GMT LOG:  database system is ready to accept connections
2017-08-16 03:46:04 GMT LOG:  autovacuum launcher started

what could be the reason?

Configuration details:

OS: Ubuntu 16.04.1 LTS (Windows Azure VM)
PostgreSQL version: 9.5.8
listen address: * (my bad, but Azure have to block another ports from external access.
                   I didn't enable PG 5432 port for external access) 
HBA config:
local   all             postgres                                ident
host    all             all             127.0.0.1/32            md5
host    all             all             ::1/128                 md5
host    all             all             10.0.0.0/24             md5
host    replication     app_replicator 10.0.0.5/32              md5
local   all             all                                     peer

In my case, this happened automatically due to the system was out of memory.

Found logs about oom_reaper and dotnet oom tasks in Syslog, but not from Postgres. I only got the message "received fast shutdown request" in the Postgres csvlog.

PS: Since my Postgres is running inside of Docker i can't mutually exclude that Docker sent a signal and forced this shutdown (but I don't assume this)

StackExchange Database Administrators Q#183633, answer score: 6