PHP CLI image instruction adding crontab entry cannot succeed

Nobody has claimed this yet.

### Preconditions

1. Docker 20.10.12
2. Magento Cloud Docker 1.3.2 (source code)

### Steps to reproduce

1. Start in directory with [source code of Magento Cloud Docker 1.3.2](https://github.com/magento/magento-cloud-docker/releases/tag/1.3.2).
2. `cd images/php/7.4-cli`
3. `docker build .`

### Expected result

1. The relevant step succeeds:

Step 44/48 : RUN if [ ! -z "${CRONTAB}" ]; then echo "${CRONTAB}" > /etc/cron.d/magento && touch /var/log/cron.log ; fi
---> Running in f74f6ae577c2
Removing intermediate container f74f6ae577c2
---> 7f64efe9445c

Notice the difference from current Dockerfile. Expected `RUN` instruction uses POSIX-compliant `[` instead of Bash-specific `[[`.

2. Alternatively, this instruction is deemed redundant as nobody noticed since its introduction in d78c326 (and being part of two releases).

### Actual result

1. The relevant step technically succeeds, but logically always fails no matter the actual value of `$CRONTAB`:

Step 44/48 : RUN if [[ ! -z "${CRONTAB}" ]]; then echo "${CRONTAB}" > /etc/cron.d/magento && touch /var/log/cron.log ; fi
---> Running in cc4ba5f26f15
/bin/sh: 1: [[: not found
Removing intermediate container cc4ba5f26f15
---> a329b4cfebf5

Start in images/php/7.4-cli and inspect the Dockerfile instruction used during docker build. Reproduce the failure with Docker 20.10.12, then verify that the CRONTAB-related step no longer reports the shell syntax error and that the image build succeeds.

Written by the indexing model from the issue text.