omg run/subscribe shouldn't try to rebuild the docker image if run multiple times

[wilzbach]

While docker build is cached, there's still no need to go through the entire process of building the image for multiple invocations of omg run.

tl;dr: omg run will always result in the following:

> omg run hmac -a method=sha1 -a "data=hello world" -a "secret=my secret"
ℹ Building Docker image
Sending build context to Docker daemon  238.8MB
Step 1/9 : FROM dlang2/ldc-ubuntu:1.14.0 as builder
 ---> 5961d5b8c51d
Step 2/9 : WORKDIR /src
 ---> Using cache
 ---> cfd36685387e
Step 3/9 : COPY dub.sdl dub.selections.json /src/
 ---> Using cache
 ---> e7453d3921f8
Step 4/9 : RUN dub build -b release || true
 ---> Using cache
 ---> 71d48349e922
Step 5/9 : COPY source /src/source
 ---> Using cache
 ---> 59400c680683
Step 6/9 : RUN dub build -b release
 ---> Using cache
 ---> c01fff604316
Step 7/9 : FROM busybox
 ---> 59788edf1f3e
Step 8/9 : COPY --from=builder /src/hashes /hashes
 ---> Using cache
 ---> e3b74bc10ecf
Step 9/9 : ENTRYPOINT [ "/hashes" ]
 ---> Using cache
 ---> b4b0ee47f358
Successfully built b4b0ee47f358
Successfully tagged omg/wilzbach/hashes:latest
✔ Built Docker image with name: omg/wilzbach/hashes
✔ Started Docker container: c858d5b1f039
✔ Health check passed
✔ Ran action: `hmac` with output: {
  "method": "sha1",
  "digest": "9F60EE4B05E590A7F3FAC552BFB9D98FA46F78D9"
}
✔ Stopped Docker container: c858d5b1f039

On consecutive runs, instead of building the docker image again, it could directly start the docker container.