My `/etc/apt/sources.list` for Discourse in Debian 9 (Stretch)

# The official Debian repository
deb stable main contrib non-free
# The Debian source code.
# It could be needed to compile something manually.
# E.g., I have used it to compile PHP and PHP extensions manually.
# I do not know whether it is really required for a Discourse-only server.
deb-src stable main contrib non-free

# Debian updates
deb stretch-updates main contrib non-free
deb-src stretch-updates main contrib non-free

# Debian security updates
deb stretch/updates main contrib non-free
deb-src stretch/updates main contrib non-free

# Debian proposed updates
deb stretch-proposed-updates main contrib non-free

# For Docker
# 2017-04-29
# Now it is used to install Certbot too.
# I use Certbot to install and automatically update my Let's Encrypt certificates.
deb stretch-backports main
# 2017-10-29 For Docker:
deb [arch=amd64] stretch stable
deb-src [arch=amd64] stretch stable