docs: Add design document for artifact reproducibility

Refs #1047

docs/developer/reproducibility.md

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+# Reproducible builds
+
+We want to improve the transparency and auditability of our build artifacts, and
+a way to achieve this is via reproducible builds. For a broader understanding of
+what reproducible builds entail, check out https://reproducible-builds.org/.
+
+Our build artifacts consist of:
+* Container images (`amd64` and `arm64` architectures)
+* macOS installers (for Intel and Apple Silicon CPUs)
+* Windows installer
+* Fedora packages (for regular Fedora distros and Qubes)
+* Debian packages (for Debian and Ubuntu)
+
+As of writing this, only the following artifacts are reproducible:
+* Container images (see [#1047](https://github.com/freedomofpress/dangerzone/issues/1047))
+
+In the following sections, we'll mention some specifics about enforcing
+reproducibility for each artifact type.
+
+## Container image
+
+### Updating the image
+
+The fact that our image is reproducible also means that it's frozen in time.
+This means that rebuilding the image without updating our Dockerfile will
+**not** receive security updates.
+
+Here are the necessary variables that make up our image in the `Dockerfile.env`
+file:
+* `DEBIAN_IMAGE_DATE`: The date that the Debian container image was released
+* `DEBIAN_ARCHIVE_DATE`: The Debian snapshot repo that we want to use
+* `GVISOR_ARCHIVE_DATE`: The gVisor APT repo that we want to use
+* `H2ORESTART_CHECKSUM`: The SHA-256 checksum of the H2ORestart plugin
+* `H2ORESTART_VERSION`: The version of the H2ORestart plugin
+
+If you update these values in `Dockerfile.env`, you must also create a new
+Dockerfile with:
+
+```
+make Dockerfile
+```
+
+Updating `Dockerfile` without bumping `Dockerfile.in` is detected and should
+trigger a CI error.
+
+### Reproducing the image
+
+For a simple way to reproduce a Dangerzone container image, you can checkout the
+commit this image was built from (you can find it from the image tag in its
+`g<commit>` portion), and run the following command in a Linux environment:
+
+```
+./dev_scripts/reproduce-image.py --source <image>
+```
+
+This command will download the `diffoci` helper, build a container image from
+the current Git commit, and ensure that the built image matches the source one,
+with the exception of image names and file timestamps.
+
+> [!TIP]
+> If the source image is not pushed to a registry, and is local instead, you
+> can prefix it with `docker://` or `podman://` accordingly, so that `diffoci`
+> can load it from the local Docker / Podman container engine. For example:
+>
+> ```
+> ./dev_scripts/reproduce.py --source podman://dangerzone.rocks/dangerzone:0.8.0-125-g725ce3b
+> ```