Cloudbuild - build docker image with custom variable from a different step - google-cloud-build

Question

I want to achieve the following build process:
decide the value of environment var depending on the build branch
persist this value through diff build steps
use this var to pass it as build-arg to docker build
Here is some of the cloudbuild config I've got:
- id: 'Get env from branch'
name: bash
args:
- '-c'
- |-
environment="dev"
if [[ "${BRANCH_NAME}" == "staging" ]]; then
environment="stg"
elif [[ "${BRANCH_NAME}" == "master" ]]; then
environment="prd"
fi
echo $environment > /workspace/environment.txt
- id: 'Build Docker image'
name: bash
dir: $_SERVICE_DIR
args:
- '-c'
- |-
environment=$(cat /workspace/environment.txt)
echo "===== ENV: $environment"
docker build --build-arg ENVIRONMENT=$environment -t gcr.io/${_GCR_PROJECT_ID}/${_SERVICE_NAME}/${COMMIT_SHA} .
The problem lies in the 2nd step. If I use bash step image, then I've got no docker executable in order to build my custom image.
And if I use gcr.io/cloud-builders/docker step image, then I can't execute bash scripts. In the args field I can only pass arguments for the docker executable. And this way I cannot extract the value of environment that I've persisted through the steps of the build.
The way I managed to accomplish both is to use my own, custom, pre-built image, which contains both bash and docker executables. I have that image in the container registry and I use it as the build step image. But this requires some custom work from my side. I was wondering if there is a better, more standardized way with built-in tools from cloudbuild.
Sources:
how to run inline bash scripts
how to persist values through build steps

Answer 1

You can change the default entrypoint by adding entrypoint: parameter
- name: 'gcr.io/cloud-builders/docker'
entrypoint: 'bash'
args:
- -c
- |
echo $PROJECT_ID
environment=$(cat /workspace/environment.txt)
echo "===== ENV: $environment"
docker build --build-arg ENVIRONMENT=$environment -t gcr.io/${_GCR_PROJECT_ID}/${_SERVICE_NAME}/${COMMIT_SHA} .