Fix running containers that don't have shell entrypoints

Created by: mrnugget

Previously when trying to run the following spec step:

steps:
  - run: comby -in-place 'fmt.Sprintf("%d", :[v])' 'strconv.Itoa(:[v])' .go -matcher .go -d /work -exclude-dir .,vendor
    container: comby/comby

I got this error:

executing campaign spec: Error executing task. The log file can be found at:
  /tmp/changeset-github.com-sourcegraph-automation-testing.log747594520
  : probing image "sha256:c7bdf774c287814a4958bd175f54e598571eae161aea791df4b62e255df6f590" for shell: 2 errors occurred:
       * probing shell "/bin/bash":
  Error parsing command line:

    flag -c is an ambiguous prefix: -color, -count, -custom-matcher

  For usage information, run

    comby -help

  : exit status 1
       * probing shell "/bin/sh":
  Error parsing command line:

    flag -c is an ambiguous prefix: -color, -count, -custom-matcher

  For usage information, run

    comby -help

  : exit status 1

This fixes that error (and it fixes https://github.com/sourcegraph/sourcegraph/issues/13620) by setting the --entrypoint on each container and then passing arguments to that.