Build system for hub.docker.com

DOCKER_HUB.md

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+# cloud.docker.com Configuration
+
+The automatic build is configured in cloud.docker.com.
+
+The following build configuration is expected:
+
+```yaml
+Source Repository: github.com/netbox-community/netbox-docker
+Build Location: Build on Docker Hub's infrastructure
+Autotest: Internal and External Pull Requests
+Repository Links: Enable for Base Image
+Build Rules:
+- Source Type: Branch
+  Source: master
+  Docker Tag: branches-main
+  Dockerfile location: Dockerfile
+- Source Type: Branch
+  Source: master
+  Docker Tag: branches-ldap
+  Dockerfile location: Dockerfile.ldap
+- Source Type: Branch
+  Source: master
+  Docker Tag: prerelease-main
+  Dockerfile location: Dockerfile
+- Source Type: Branch
+  Source: master
+  Docker Tag: prerelease-ldap
+  Dockerfile location: Dockerfile.ldap
+- Source Type: Branch
+  Source: master
+  Docker Tag: release-main
+  Dockerfile location: Dockerfile
+- Source Type: Branch
+  Source: master
+  Docker Tag: release-ldap
+  Dockerfile location: Dockerfile.ldap
+Build Environment Variables:
+# Create an app on Github and use it's OATH credentials here
+- Key: GITHUB_OAUTH_CLIENT_ID
+  Value: <secret>
+- Key: GITHUB_OAUTH_CLIENT_SECRET
+  Value: <secret>
+Build Triggers:
+- Name: Cron Trigger
+# Use this trigger in combination with e.g. https://cron-job.org in order to regularly schedule builds
+```
+
+## Background Knowledge
+
+The build system of cloud.docker.com is not made for this kind of project.
+But we found a way to make it work, and this is how:
+
+1. The docker hub build system [allows to overwrite the scripts that get executed
+   for `build`, `test` and `push`](overwrite). See `hooks/*`.
+2. Shared functionality of the scripts `build`, `test` and `push` is extracted to `hooks/common`.
+3. The `build` script runs `run_build()` from `hooks/common`.
+   This triggers either `build-branches.sh`, `build-latest.sh` or directly `build.sh`.
+4. The `test` script just invokes `docker-compose` commands.
+5. The `push` script runs `run_build()` from `hooks/common` with a `--push-only` flag.
+   This causes the `build.sh` script to not re-build the Docker image, but just the just built image.
+
+The _Docker Tag_ configuration setting is misused to select the type (_release_, _prerelease_, _branches_) of the build as well as the variant (_main_, _ldap_).
+
+The _Dockerfile location_ configuration setting is completely ignored by the build scripts.
+
+[overwrite]: https://docs.docker.com/docker-hub/builds/advanced/#override-build-test-or-push-commands