Dockerfile.local: place COPY targets under /out

[eriknordmark]

Summary

make image-local (documented in README.md as the recommended quick
reproducible build path) currently produces an image without
/bin/adam. Running such an image fails immediately with:

"/bin/adam": stat /bin/adam: no such file or directory

Root cause

Dockerfile.local's build stage sets WORKDIR /out but the three
COPY directives target absolute paths at root:

WORKDIR /out
COPY scripts /bin
COPY samples /adam
COPY ./bin/adam-linux-amd64 /bin/adam

Absolute paths in COPY are not joined with WORKDIR, so the
files land at /bin, /adam, and /bin/adam in the build stage's
filesystem.

The final scratch stage then does:

FROM scratch
COPY --from=build /out/ /

…which copies only what's under /out/. The scripts, samples, and
adam binary at the build-stage root are silently dropped from the
final image.

The non-.local Dockerfile (used for CI / Docker Hub builds) does
this correctly — RUN go build -o /out/bin/adam main.go,
COPY scripts/ /out/bin/, COPY samples/ /out/adam/. Only
Dockerfile.local has the bug.

Fix

Redirect the three COPY destinations to /out/bin/, /out/adam/,
and /out/bin/adam so the scratch stage picks them up.

How to test

Before this patch, on master HEAD:

make build                 # produce bin/adam-linux-amd64
make image-local IMG=lfedge/adam:test
docker run --rm --entrypoint=/bin/sh lfedge/adam:test -c 'ls /bin/adam'
# → ls: /bin/adam: No such file or directory

After this patch:

docker run --rm --entrypoint=/bin/sh lfedge/adam:test -c 'ls -la /bin/adam'
# → -rwxr-xr-x 1 root root 21998444 … /bin/adam
docker run --rm --entrypoint=/bin/sh lfedge/adam:test -c 'ls /adam/'
# → simple.json … (samples present)

The actual make image-local flow (which calls make build first on
my local machine has an unrelated go.mod-version vs. pinned-golang
issue), but the Dockerfile change itself is independently verifiable
by building it directly with a pre-existing bin/adam-linux-amd64,
which I've done locally as shown above.