nixpkgs/pkgs/build-support/fetchdocker/credentials.nix

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# We provide three paths to get the credentials into the builder's
# environment:
#
# 1. Via impureEnvVars. This method is difficult for multi-user Nix
# installations (but works very well for single-user Nix
# installations!) because it requires setting the environment
# variables on the nix-daemon which is either complicated or unsafe
# (i.e: configuring via Nix means the secrets will be persisted
# into the store)
#
# 2. If the DOCKER_CREDENTIALS key with a path to a credentials file
# is added to the NIX_PATH (usually via the '-I ' argument to most
# Nix tools) then an attempt will be made to read credentials from
# it. The semantics are simple, the file should contain two lines
# for the username and password based authentication:
#
# $ cat ./credentials-file.txt
# DOCKER_USER=myusername
# DOCKER_PASS=mypassword
#
# ... and a single line for the token based authentication:
#
# $ cat ./credentials-file.txt
# DOCKER_TOKEN=mytoken
#
# 3. A credential file at /etc/nix-docker-credentials.txt with the
# same format as the file described in #2 can also be used to
# communicate credentials to the builder. This is necessary for
# situations (like Hydra) where you cannot customize the NIX_PATH
# given to the nix-build invocation to provide it with the
# DOCKER_CREDENTIALS path
let
pathParts =
(builtins.filter
({prefix, path}: "DOCKER_CREDENTIALS" == prefix)
builtins.nixPath);
in
if (pathParts != []) then (builtins.head pathParts).path else ""