Installation: Add Release-Via-Docker option

installation/release-to-docker/README.md

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+# Run OTP releases on older glibc using Docker
+
+Pleroma OTP releases are built on specific distros and may require a newer `glibc`
+than your host has. A typical failure looks like:
+
+```
+... /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.38' not found ...
+```
+
+If you don't want to upgrade your host OS, you can run the existing OTP release
+from `/opt/pleroma` inside an Ubuntu 24.04 container while keeping your existing
+host paths (`/etc/pleroma`, `/var/lib/pleroma`, etc.).
+
+This folder provides a "shim" container + systemd unit. It is **not** the Pleroma
+Docker image.
+
+## What this does
+
+- Builds a small Ubuntu 24.04 image with runtime libs (including newer `glibc`).
+- Mounts your existing host release at `/opt/pleroma` into the container.
+- Runs as the same UID/GID that owns `/opt/pleroma` on the host (via `gosu`).
+- Optionally runs migrations automatically on container start.
+- Uses `network_mode: host` so your existing config that talks to `localhost`
+  keeps working.
+
+## Setup (Debian/Ubuntu host)
+
+1. Install Docker Engine + the Docker Compose plugin.
+2. Copy these files to a stable location (example: `/etc/pleroma/container`):
+
+   ```
+   mkdir -p /etc/pleroma/container
+   cp -a /opt/pleroma/installation/release-to-docker/* /etc/pleroma/container/
+   ```
+
+3. Build the shim image:
+
+   ```
+   cd /etc/pleroma/container
+   docker compose build
+   ```
+
+4. Replace your systemd unit:
+
+   ```
+   cp /etc/pleroma/container/pleroma.service /etc/systemd/system/pleroma.service
+   systemctl daemon-reload
+   systemctl enable --now pleroma
+   journalctl -u pleroma -f
+   ```
+
+## Running `pleroma_ctl`
+
+Since the host binary may not run on older `glibc`, run admin commands inside the
+container:
+
+```
+cd /etc/pleroma/container
+docker compose exec pleroma /opt/pleroma/bin/pleroma_ctl status
+docker compose run --rm --no-deps pleroma /opt/pleroma/bin/pleroma_ctl migrate
+```
+
+## Configuration notes
+
+- Migrations run automatically by default.
+  - Set `PLEROMA_RUN_MIGRATIONS=0` in `docker-compose.yml` to disable.