Proposal: Support secrets mounted as files

[vladaionescu]

Currently secrets are env vars in Earthly. There are many situations where files are preferable.

[alexcb]

This is being worked on in #654

[alexcb]

Here's a working example of this:

Earthfile:

FROM alpine:latest

mytarget:
    RUN --mount type=secret,id=+secrets/key,target=/mykey echo "the secret key is $(cat /mykey)"

example run:

$ earthly --no-cache --secret key="cret cret cret cret cret" +mytarget
           buildkitd | Found buildkit daemon as docker container (earthly-buildkitd)
       alpine:latest | --> Load metadata linux/amd64
               +base | --> FROM alpine:latest
               +base | [██████████] resolve docker.io/library/alpine:latest@sha256:69e70a79f2d41ab5d637de98c1e0b055206ba40a8145e7bddb55ccc04e13cf8f ... 100%
           +mytarget | --> RUN echo "the secret key is $(cat /mykey)"
           +mytarget | the secret key is cret cret cret cret cret
              output | --> exporting outputs
              output | [██████████] sending tarballs ... 100%
=================================== SUCCESS ====================================

[vladaionescu]

Wonder if such an example should be shown in our docs somewhere too.

[vladaionescu]

Needs docs