pure-docker: add postgres configuration file mounts

[kevinwojo]

Bind mount the postgres configuration file to allow admins to tune their databases in the pure-docker deployment.

See: https://github.com/sourcegraph/customer/issues/854

Checklist

• [x] Sister deploy-sourcegraph change:
• [x] All images have a valid tag and SHA256 sum

Test plan

Manual testing

manual test result

successful creation of mount

docker exec -it pgsql bash -c 'cat /etc/mtab | grep "/conf"'
grpcfuse /conf fuse.grpcfuse rw,nosuid,nodev,relatime,user_id=0,group_id=0,allow_other,max_read=1048576 0 0

correct file copied into postgres data dir

docker exec -it pgsql bash -c 'cat /var/lib/postgresql/data/pgdata/postgresql.conf'
#------------------------------------------------------------------------------
# POSTGRESQL DEFAULT CONFIGURATION
#------------------------------------------------------------------------------

# Below is PostgreSQL default configuration.
# You should apply your own customization in the CUSTOMIZED OPTIONS section below
# to avoid merge conflict in the future.

listen_addresses = '*'
max_connections = 100
shared_buffers = 128MB
dynamic_shared_memory_type = posix
max_wal_size = 1GB
min_wal_size = 80MB
log_timezone = 'UTC'
datestyle = 'iso, mdy'
timezone = 'UTC'
lc_messages = 'en_US.utf8'
lc_monetary = 'en_US.utf8'
lc_numeric = 'en_US.utf8'
lc_time = 'en_US.utf8'
default_text_search_config = 'pg_catalog.english'


#------------------------------------------------------------------------------
# SOURCEGRAPH RECOMMENDED OPTIONS
#------------------------------------------------------------------------------

# Below is Sourcegraph recommended Postgres configuration based on the default resource configuration.
# You should apply your own customization in the CUSTOMIZED OPTIONS section below
# to avoid merge conflict in the future.

shared_buffers = 509546kB
work_mem = 3184kB
maintenance_work_mem = 254773kB
effective_io_concurrency = 200
max_worker_processes = 19
max_parallel_workers_per_gather = 4
max_parallel_workers = 8
wal_buffers = 15285kB
min_wal_size = 512MB
checkpoint_completion_target = 0.9
random_page_cost = 1.1
effective_cache_size = 1492MB
default_statistics_target = 500
autovacuum_max_workers = 10
autovacuum_naptime = 10
shared_preload_libraries = ''
max_locks_per_transaction = 64


#------------------------------------------------------------------------------
# CUSTOMIZED OPTIONS
#------------------------------------------------------------------------------

# Add your customization here
# Learn more: https://docs.sourcegraph.com/admin/config/postgres-conf