Add Dockerfiles

Would be very useful to have scratch container images with the binaries included in them.

An excellent use case is Alpine linux: it does not ship with an envsubst. A Docker image would allow copying an arch specific binary over:

```dockerfile
FROM alpine:edge
COPY --from=ghcr.io/a8m/envsubst /envsubst /envsubst
```

The project builds a Go binary. Look at the existing build process, likely in a Makefile or go build commands. Create Dockerfiles for scratch images that copy the compiled binary. Test by building the image and verifying the binary runs. Check if multi-arch builds are needed.

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