Introduce reproducible, prebuilt binaries

Dockerfile

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+# Building this Dockerfile executes a full build of the Recovery Tool, cross-compiling inside 
+# the container. The resulting executable is copied to the output directory using Docker BuildKit.
+
+# You need to pass 3 parameters via --build-arg:
+# 1. `os`  : the GOOS env var -- `linux`, `windows` or `darwin`.
+# 2. `arch`: the GOARCH env var -- `386` or `amd64` (note that darwin/386 is not a thing).
+# 3. `out` : the name of the resulting executable, placed in the output directory on the host.
+
+# For example, to build a linux/386 binary into `bin/rt`:
+#   docker build . --output bin --build-arg os=linux --build-arg arch=386 --build-arg out=rt
+
+# Note that the --output <dir> flag refers to the host, outside the container.
+
+FROM golang:1.16.0-alpine3.13 AS build
+ARG os
+ARG arch
+
+RUN apk add --no-cache build-base=0.5-r2
+
+WORKDIR /src
+COPY . .
+
+ENV CGO_ENABLED=0
+RUN env GOOS=${os} GOARCH=${arch} go build -mod=vendor -a -trimpath -o /out .
+
+# ---
+
+FROM scratch
+ARG out
+
+COPY --from=build /out ${out}