Github action running on docker image not recognising if [[...]] statements

I've got an action that runs on PR's that are going from "release/**" into master that incriments the RC version each time it runs and commits it back into the release branch.

To stop a loop starting, I have the following step

- name: Check if version number has changed
        run: |
          if [[ $(git show HEAD | grep -e "\"version\":.*-rc\..*") ]]; then
            echo "Version number has changed, moving onto next jobs"
            exit 1
          else
            echo "Version number has not changed, continuing with this job"
          fi

However, when running, it just responds with

1: [[: not found
Version number has not changed, continuing with this job

With the result being the infinite loop that I'm trying to avoid.

This was initially running on a node:lts container, but I switched it to ubuntu:latest when I started hitting this issue to try and remedy it, to no avail. I've also tried adding #!/bin/bash to the top of the step.

How can I get around this? The if statement works perfectly when ran locally on my WSL ubuntu install.

As @jessehouwing said to do, running with shell: bash fixed the issue

  - name: Check if version number has changed
    shell: bash
    run: |
      git show HEAD | grep -e "\"version\":.*-rc\..*"
      if [[ $(git show HEAD | grep -e "\"version\":.*-rc\..*") ]]; then
        echo "Version number has changed, moving onto next jobs"
        echo "SKIP_PRE_RELEASE=true" >> $GITHUB_ENV
      else
        echo "Version number has not changed, continuing with this job"
      fi

This is the working form of the above step.