docs: describe "make docker-release"

Author: starius

README.md

@@ -125,3 +125,8 @@ git clone https://github.com/lightninglabs/loop.git
 cd loop/cmd
 go install ./...
 ```
+
+## Reproducible builds
+
+If you want to build release files yourself, follow
+[the guide](./docs/release.md).

docs/release.md

@@ -0,0 +1,81 @@
+# Reproducible Builds
+
+## Building with Docker
+
+To create a Loop release with binaries that are identical to an official
+release, run the following command (available since release `v0.31.3-beta`):
+
+```bash
+make docker-release tag=<tag-of-release>
+```
+
+This command will create a directory named `loop-<tag-of-release>` containing
+the source archive, vendored dependencies, and the built binaries packaged in
+`.tar.gz` or `.zip` format. It also creates a manifest file with `SHA-256`
+checksums for all release files.
+
+For example:
+
+```bash
+make docker-release tag=v0.31.3-beta
+```
+
+This will create the release artifacts in the `loop-v0.31.3-beta` directory.
+
+If you want to build from an untagged commit, first check it out, then use the
+output of `git describe` as the tag:
+
+```bash
+git describe
+# v0.31.2-beta-128-gfa80357
+
+make docker-release tag=v0.31.2-beta-128-gfa80357
+```
+
+You can filter the target platforms to speed up the build process. For example,
+to build only for `linux-amd64`:
+
+```bash
+make docker-release buildsys=linux-amd64 tag=v0.31.3-beta
+```
+
+Or for multiple platforms:
+
+```bash
+make docker-release buildsys='linux-amd64 windows-amd64' tag=v0.31.3-beta
+```
+
+Note: inside Docker the current directory is mapped as `/repo` and it might
+mention `/repo` as parts of file paths.
+
+## Building on the Host
+
+You can also build a release on your host system without Docker. You will need
+to install the Go version specified in the `go.mod` file, as well as a few
+other tools:
+
+```bash
+sudo apt-get install build-essential git make zip perl gpg
+```
+
+Add GPG key of Alex Bosworth to verify release tag signature:
+```bash
+gpg --keyserver keys.openpgp.org --recv-keys DE23E73BFA8A0AD5587D2FCDE80D2F3F311FD87E
+```
+
+Then, run the `release.sh` script directly:
+
+```bash
+./release.sh <tag-of-release>
+```
+
+To filter the target platforms, pass them as a space-separated list in the
+`LOOPBUILDSYS` environment variable:
+
+```bash
+LOOPBUILDSYS='linux-amd64 windows-amd64' ./release.sh v0.31.3-beta
+```
+
+This will produce the same artifacts in a `loop-<tag-of-release>` directory as
+the `make docker-release` command. The latter simply runs the `release.sh`
+script inside a Docker container.