Docker build secret utility.

This project provides a simple tool to pass secrets to docker build.
It does so while trying to minimize the exposure of sensitive information.

Important: This tool requires --network=host to be specified in the docker build arguments.

wget -P /usr/local/bin https://github.com/3XX0/donkey/releases/download/v1.1.0/donkey
chmod +x /usr/local/bin/donkey

donkey set <filename> [<count>]

Create a one-time secret (by default) and display its identifier on standard output.
filename must point to either the path of the secret or - for standard input. In the case where count is specified, the secret will be made available exactly n-times instead, the exception being that it can not be used with the standard input modifier.

donkey get <id> [<command>]

Retrieve a secret from an identifier and display its content on standard output.
When a command is given, the secret is available through the file refered by the DONKEY_FILE environment variable instead.

docker build --network=host
             --build-arg FOO=$(cat secret.txt | donkey set - &)
             --build-arg BAR=$(donkey set secret.txt 2 &)
             -f Dockerfile.sample .

Without Docker:

make

With Docker:

make docker

1. The command donkey set creates an abstract unix socket (see unix(7)) and displays its address on standard output. It then waits for an incoming connection from root:root.

2. The command donkey get connects to the above address from within the docker build context, thus establishing a communication channel crossing the container boundary. Note that this is only possible with --network=host since the network namespace would isolate the abstract socket.

3. The secret file is then mapped into memory and sent through the socket using splice(2)

4. An unnamed temporary file is created inside the container under /dev/shm to store the secret. This file is advertized through the environment variable DONKEY_FILE and remains valid for the duration of the command specified in the donkey get arguments (sh -c in the example).