After every change to your formulae, perform the following from the root of the Git repository (not from support/build/_docker/):
$ docker build --tag heroku-php-build-cedar-14 --file $(pwd)/support/build/_docker/cedar-14.Dockerfile .
File env.default contains a list of required env vars, some with default values. You can modify it with the values you desire, or pass them to docker run using --env.
Out of the box, you'll likely want to change S3_BUCKET and S3_PREFIX to match your info. Instead of setting AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY in that file, it is recommended to pass them to docker run through the environment, or explicitly using --env, in order to prevent accidental commits of credentials.
From the root of the Git repository (not from support/build/_docker/):
docker run --rm --tty --interactive --env-file=support/build/_docker/env.default heroku-php-build-cedar-14 /bin/bash
That runs with values from env.default; if you need to pass e.g. AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY because they are not already in your environment, do either:
AWS_ACCESS_KEY_ID=... AWS_SECRET_ACCESS_KEY=... docker run --rm --tty --interactive --env-file=support/build/_docker/env.default heroku-php-build-cedar-14 /bin/bash
or
docker run --rm --tty --interactive --env-file=support/build/_docker/env.default -e AWS_ACCESS_KEY_ID=... -e AWS_SECRET_ACCESS_KEY=... heroku-php-build-cedar-14 /bin/bash
You then have a shell where you can run bob build, deploy.sh and so forth.
The support/build/_util/ directory is on $PATH in the image.