Use an existing or a new Docker image
Add the fwatchdog binary from the Releases page via curl or ADD https://
Set an fprocess environmental variable with the function you want to run for each request
Expose port 8080
Set the CMD to fwatchdog

Example Dockerfile:

FROM alpine:3.5

ADD https://github.com/alexellis/faas/releases/download/v0.5-alpha/fwatchdog /usr/bin
RUN chmod +x /usr/bin/fwatchdog

# Define your UNIX binary here
ENV fprocess="/bin/cat"

CMD ["fwatchdog"]

Implementing the a healthcheck

Docker swarm will keep your function out of the DNS-RR / IPVS pool if the task (container) is not healthy.

Here is an example of the echo function implementing a healthcheck with a 5-second checking interval.

FROM functions/alpine

ENV fprocess="cat /etc/hostname"

HEALTHCHECK --interval=5s CMD [ -e /tmp/.lock ] || exit 1

The watchdog process creates a .lock file in /tmp/ on starting its internal Golang HTTP server. [ -e file_name ] is shell to check if a file exists.

Swarm tutorial on Healthchecks:

Test-drive Docker Healthcheck in 10 minutes

Environmental overrides:

A number of environmental overrides can be added for additional flexibility and options:

Option	Usage
`fprocess`	The process to invoke for each function call. This must be a UNIX binary and accept input via STDIN and output via STDOUT.
`marshal_requests`	Instead of re-directing the raw HTTP body into your fprocess, it will first be marshalled into JSON. Use this if you need to work with HTTP headers
`write_timeout`	HTTP timeout for writing a response body from your function
`read_timeout`	HTTP timeout for reading the payload from the client caller
`suppress_lock`	The watchdog will attempt to write a lockfile to /tmp/ for swarm healthchecks - set this to true to disable behaviour.