do not generate dockerfiles with ENV DEPS being empty

[avsm]

This results in an error in dockerfile parsing, e.g. from https://ci.ocamllabs.io:8100/job/2020-07-24/004716-ci-ocluster-build-f8ddc7

git clone --recursive "https://github.com/favonia/yuujinchou.git" && cd "yuujinchou" && git fetch origin "refs/pull/34/head" && git reset --hard 62d123e5
cat > Dockerfile <<'END-OF-DOCKERFILE'
FROM ocurrent/opam@sha256:84a5debe280b7aded6828f9be21bd0eb82d03c850faa6935ef762d4665e6ead1
# debian-10-ocaml-4.03
WORKDIR /src
RUN sudo chown opam /src
RUN cd ~/opam-repository && (git reset --hard 1a576beaaa094013d2935556a76bf48f89ae6098 || (git pull origin master && git reset --hard 1a576beaaa094013d2935556a76bf48f89ae6098)) && opam update -u
RUN mkdir -p "./"
COPY yuujinchou.opam ./
RUN opam pin add -yn yuujinchou.dev "./"
ENV DEPS 
RUN opam depext --update -y yuujinchou.dev $DEPS
RUN opam install $DEPS
COPY --chown=opam . /src/
RUN opam exec -- dune build @install @runtest && rm -rf _build
END-OF-DOCKERFILE
docker build .

2020-07-24 00:47.16: Using cache hint "favonia/yuujinchou-ocurrent/opam@sha256:84a5debe280b7aded6828f9be21bd0eb82d03c850faa6935ef762d4665e6ead1-debian-10-ocaml-4.03-b7c2a91365c1f6898231aa5a71eac69a"
2020-07-24 00:47.16: Waiting for resource in pool OCluster
2020-07-24 00:47.16: Waiting for worker...
2020-07-24 00:47.16: Got resource from pool OCluster
Using BuildKit Dockerfile:
# syntax = docker/dockerfile:experimental@sha256:ee85655c57140bd20a5ebc3bb802e7410ee9ac47ca92b193ed0ab17485024fe5
FROM ocurrent/opam@sha256:84a5debe280b7aded6828f9be21bd0eb82d03c850faa6935ef762d4665e6ead1
# debian-10-ocaml-4.03
WORKDIR /src
RUN sudo chown opam /src
RUN cd ~/opam-repository && (git reset --hard 1a576beaaa094013d2935556a76bf48f89ae6098 || (git pull origin master && git reset --hard 1a576beaaa094013d2935556a76bf48f89ae6098)) && opam update -u
RUN mkdir -p "./"
COPY yuujinchou.opam ./
RUN opam pin add -yn yuujinchou.dev "./"
ENV DEPS 
RUN --mount=type=cache,target=/home/opam/.opam/download-cache,uid=1000 opam depext --update -y yuujinchou.dev $DEPS
RUN --mount=type=cache,target=/home/opam/.opam/download-cache,uid=1000 opam install $DEPS
COPY --chown=opam . /src/
RUN opam exec -- dune build @install @runtest && rm -rf _build
Building on x86-build-5.ocamllabs.io
All commits already cached
HEAD is now at 62d123e Add the efficient Trie.to_seq_values.
#2 [internal] load .dockerignore
#2 sha256:014d103dfe70ba646e0c923b243b14440f2a099d89a55012ddc9dc66266508d7
#2 transferring context: 0.0s
#2 transferring context: 2B 0.0s done
#2 ...

#1 [internal] load build definition from Dockerfile
#1 sha256:3e3bf8d7e84687460518e505107ec466b05eca59241bcef40fb7c66a2b54bca0
#1 transferring dockerfile: 887B 0.0s done
#1 DONE 0.4s

#2 [internal] load .dockerignore
#2 sha256:014d103dfe70ba646e0c923b243b14440f2a099d89a55012ddc9dc66266508d7
#2 DONE 0.5s

#4 docker-image://docker.io/docker/dockerfile:experimental@sha256:ee85655c57140bd20a5ebc3bb802e7410ee9ac47ca92b193ed0ab17485024fe5
#4 sha256:71a594c684f98692da7baadc89226dfbe7a87a365660bd1251c8db6c1d7f4620
#4 CACHED

#3 resolve image config for docker.io/docker/dockerfile:experimental@sha256:ee85655c57140bd20a5ebc3bb802e7410ee9ac47ca92b193ed0ab17485024fe5
#3 sha256:8db1d80318d235996b816ce713bcf8909e6fb35d7d54a0c31710b9c3b442271a
#3 DONE 0.0s
failed to solve with frontend dockerfile.v0: failed to solve with frontend gateway.v0: rpc error: code = Unknown desc = ENV must have two arguments
docker-build failed with exit-code 1
2020-07-24 00:47.34: Job failed: Failed: Build failed

[talex5]

Maybe the analysis phase should reject this. If you're writing an OCaml program, you should at least have a dependency on OCaml.

[talex5]

Actually, I don't see how this can happen. Building that commit with the current version, I get:

ENV DEPS base-bigarray.base base-threads.base base-unix.base ocaml.4.03.0 ocaml-base-compiler.4.03.0 ocaml-config.1

We always include a version of OCaml, even if the package doesn't specify any dependencies at all.

Closing as probably out-of-date. Please reopen if it's still happening.