Confusing error when missing FROM

[ohadravid]

For this Earthfile:

hello:
    RUN mkdir /foo
    COPY bar /foo

Version: earthly version v0.5.1 linux/amd64

$ earthly +hello
           buildkitd | Found buildkit daemon as docker container (earthly-buildkitd)
No ssh auth socket detected or zero keys loaded; falling back to https for auto auth values
             context | --> local context .
              +hello | --> RUN mkdir /foo
              +hello | WARN: (RUN mkdir /foo) invalid mountinput dest:"/run/secrets/earthly_debugger_settings" mountType:SECRET secretOpt:<ID:"earthly_debugger_settings" mode:292 >
             context | [          ] transferring .: ... 0%
             context | WARN: Canceled
Error: failed to load cache key: invalid mountinput dest:"/run/secrets/earthly_debugger_settings" mountType:SECRET secretOpt:<ID:"earthly_debugger_settings" mode:292 >

The fix is easy: just add the missing FROM, but the error is very unintuitive.

Thanks! 😄

[vladaionescu]

It's probably best if we don't allow any command other than FROM, FROM DOCKERFILE or LOCALLY as the first thing being interpreted. Then we can display a meaningful error if the user forgets.

This is also relevant for the IF proposal (#779), where the user may be tempted to start off with an IF (but it doesn't work).