Allow build of docker image without requirements.txt

Prior to this, we could not build for example the software-stories image (which does not have any requirements.txt).

This changes nothing for the other docker image build.

Testing within docker (with enough clicks) and it's working [1] without impacting the previous builds [2]

[1] build-docker-image software-stories (dry-run)

$ docker compose up -d
$ click click click trigger build for software-stories
Removing intermediate container 9d2842e4c313
 ---> 2652c39ae1d5
Step 8/8 : ENTRYPOINT "/opt/swh/entrypoint.sh"
 ---> Running in be6826a0bcb3
Removing intermediate container be6826a0bcb3
 ---> 91d4ba2248b5
Successfully built 91d4ba2248b5
Successfully tagged container-registry.softwareheritage.org/swh/infra/swh-apps/software_stories:20230911.2
[Pipeline] }
[Pipeline] // script
[Pipeline] }
[Pipeline] // stage
[Pipeline] stage
[Pipeline] { (Git tag & push)
[Pipeline] script
[Pipeline] {
[Pipeline] echo
Skipping `git push`.
...
+ docker image rm container-registry.softwareheritage.org/swh/infra/swh-apps/software_stories:20230911.2
Untagged: container-registry.softwareheritage.org/swh/infra/swh-apps/software_stories:20230911.2
Deleted: sha256:91d4ba2248b5eaf2911346dc9d73b8922fefdc4a2fc6f2e30125060951f8b111
...
[Pipeline] }
[Pipeline] // script
[Pipeline] cleanWs
[WS-CLEANUP] Deleting project workspace...
[WS-CLEANUP] Deferred wipeout is used...
[WS-CLEANUP] done
[Pipeline] }
[Pipeline] // stage
[Pipeline] }
[Pipeline] // node
[Pipeline] End of Pipeline
Finished: SUCCESS

[2] build-docker-image swh-vault (dry-run)

Successfully built d495b5c60a17
Successfully tagged container-registry.softwareheritage.org/swh/infra/swh-apps/scrubber:20230911.2
[Pipeline] }
[Pipeline] // script
[Pipeline] }
[Pipeline] // stage
[Pipeline] stage
[Pipeline] { (Git tag & push)
[Pipeline] script
[Pipeline] {
[Pipeline] echo
Skipping `git push`.
[Pipeline] }
[Pipeline] // script
[Pipeline] }
[Pipeline] // stage
[Pipeline] stage
[Pipeline] { (Push image in registry)
[Pipeline] script
[Pipeline] {
[Pipeline] echo
Skipping `docker image push`.
[Pipeline] }
[Pipeline] // script
[Pipeline] }
[Pipeline] // stage
[Pipeline] stage
[Pipeline] { (Declarative: Post Actions)
[Pipeline] script
[Pipeline] {
[Pipeline] sh
+ docker image rm container-registry.softwareheritage.org/swh/infra/swh-apps/scrubber:20230911.2
Untagged: container-registry.softwareheritage.org/swh/infra/swh-apps/scrubber:20230911.2
Deleted: sha256:d495b5c60a1770c2ca21f969ebe7a2b5b1825b0425c33dc48afea513f0baa78d
...
[Pipeline] }
[Pipeline] // script
[Pipeline] cleanWs
[WS-CLEANUP] Deleting project workspace...
[WS-CLEANUP] Deferred wipeout is used...
[WS-CLEANUP] done
[Pipeline] }
[Pipeline] // stage
[Pipeline] }
[Pipeline] // node
[Pipeline] End of Pipeline
Finished: SUCCESS