Multi-stage build to compile krakend's flexible configuration

Dockerfile

FROM devopsfaith/krakend as builder
ARG ENV=prod

COPY krakend.tmpl .
COPY config .

# Save temporary file to /tmp to avoid permission errors
RUN FC_ENABLE=1 \
    FC_OUT=/tmp/krakend.json \
    FC_PARTIALS="/etc/krakend/partials" \
    FC_SETTINGS="/etc/krakend/settings/$ENV" \
    FC_TEMPLATES="/etc/krakend/templates" \
    krakend check -d -t -c krakend.tmpl

FROM devopsfaith/krakend
COPY --from=builder --chown=krakend /tmp/krakend.json .

This gist uses krakend check command with -d and -t flags, meaning that after compiling the templates will show the debug of the parsed configuration and will test starting the service. Trying to start the service is safer than just validating that the template compiled, as the file can be perfectly valid but yet have conflicting routes.

Example of the assumed file structure:

.
├── config
│   ├── partials
│   ├── settings
│   │   ├── prod
│   │   │   └── env.json
│   │   └── test
│   │       └── env.json
│   └── templates
│       └── some.tmpl
├── Dockerfile
└── krakend.tmpl

Build for another environment with:
docker build --build-arg ENV=test -t krakend-test .