Add Dockerfiles

[arbourd]

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:

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

[yasn77]

@arbourd if it helps, I've created a scratch image for this -> https://github.com/yasn77/envsubst-image/pkgs/container/envsubst-image