APPUiO Secure Docker Builder


Docker builds from Dockerfiles need access to the Docker Socket and are inherently insecure. For this reason most multi-tenant container platforms do not support Docker builds. While OpenShift Container Platform, on which APPUiO is based, improves the security of builds through the use of SELinux, they are still not secure enough to run on a multi-tenant platform. Indeed we have disabled the custom build strategy (custom builders) on APPUiO for this reason.


However, since we regard building Docker images from Dockerfiles as a vital feature, APPUiO provides its own mechanism called the “APPUiO secure Docker builder” to offer this. APPUiO secure Docker builder has the following features:

  • It provides the same user experience as the OpenShift Container Platform Docker builder.
  • Builds run in virtual machines dedicated to a single APPUiO project, which in turn run on dedicated hosts, i.e. outside of APPUiO’s OpenShift Container Platform. Therefore providing full isolation between builds and customer containers as well as between builds from different customers.
  • Supports Docker cache for fast subsequent builds.
  • All communication between APPUiO’s OpenShift Container Platform and the dedicated build VMs is encrypted.
  • To compensate the loss of custom builders it provides hooks to allow users to run a script before and/or after docker build.

User-customizable builder configuration

The source secret attached to the build strategy of a build configuration can be used to configure the build. As per usual in OpenShift secrets values must be encoded using Base64.


$ oc export secrets example-source-auth
apiVersion: v1
kind: Secret
  name: example-source-auth
type: Opaque
  ssh-privatekey: LS0…Cg==
  ssh-known-hosts: Iwo=
  ssh-config: Iwo=

The string Iwo= is #\n in Base64.


Private SSH key; see OpenShift documentation.


If this attribute is set to anything, including the empty string, strict host key checking is enabled (see StrictHostKeyChecking in ssh_config(5)). The host keys for the following hosting services are already included by default:

Other host keys can be added in Base64 format. Example retrieval command:

$ ssh-keyscan | base64


SSH configuration snippet; added after the built-in options. Useful to specify different configuration options for the SSH client (i.e. the Ciphers option; see ssh_config(5)).

Build VMs

RHEL and Docker versions in the build VMs are identical the ones on APPUiOs OpenShift Container Platform.

Build Hooks

Users can add .d2i/pre_build and/or .d2i/post_build scripts to the source repository where their Dockerfile resides. The scripts

  • need to be executable and can be written in any language.
  • have access to environment variables set in the BuildConfig object, the variables documented for custom OpenShift builder images, DOCKERFILE_PATH (relative or absolute path to Dockerfile) and DOCKER_TAG (output Docker tag)
  • pre_build is executed just before docker build and has read/write to the Docker context, including the Dockerfile (use $DOCKERFILE_PATH; also passed as first argument); the output tag is given as the second argument
  • post_build is executed just after docker build and has access to the Docker context and the built image
  • are executed in the build VM as root

Build Hook Example

Here you’ll find an example which uses a pre_build script to install Maven and uses it to download a .war file from an artefact repository: The Dockerfile picks up the .war file downloaded by the pre_build script and adds to the image with an ADD instruction. In a real project the ARTIFACT environment variable would be configure in a BuildConfig. The example uses JBoss EAP, which is only available to you if you ordered it. However this approach also works with other base images.

Multi-stage builds

Note: As of September 2017 multi-stage builds are a beta feature included in the secure Docker builder.

Note: Multi-stage builds can’t be used when the source image for a build is overridden using

Docker 17.05 and newer support multi-stage builds where build stages can be partially reused for further stages. An example Dockerfile from the Docker documentation:

FROM golang:1.7.3 as builder
WORKDIR /go/src/
RUN go get -d -v
COPY app.go    .
RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o app .

FROM alpine:latest
RUN apk --no-cache add ca-certificates
WORKDIR /root/
COPY --from=builder /go/src/ .
CMD ["./app"]

Known Issues

  • The OpenShift Container Platform Docker builder exposes environment variables via an ENV instruction at the end of Dockerfile. This is not yet implemented in the APPUiO secure Docker builder.
  • Binary and image sources are currently not implemented.