Add some hardening to the build script

[PromoFaux]

What does this PR aim to accomplish?:

Drawing inspiration from: https://github.com/pi-hole/docker-pi-hole/pull/1643#discussion_r1755272752

And also from a comment where a user mentioned they could not run the build script due to missing buildx on their system. (I cannot find the comment...)

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By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code and I have tested my changes.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

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• [x] I have read the above and my PR is ready for review. Check this box to confirm

What does this PR aim to accomplish?:

---

By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code and I have tested my changes.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

---

• [x] I have read the above and my PR is ready for review. Check this box to confirm

[PromoFaux]

adam@adam-pc:~/repos/docker-pi-hole$ ./build.sh -f kjansdkajnsd
Error: FTL binary not found for branch 'kjansdkajnsd'. Exiting.
adam@adam-pc:~/repos/docker-pi-hole$ ./build.sh -c kjansdkajnsd
Error: Core branch 'kjansdkajnsd' not found. Exiting.
adam@adam-pc:~/repos/docker-pi-hole$ ./build.sh -p kjansdkajnsd
Error: PADD branch 'kjansdkajnsd' not found. Exiting.
adam@adam-pc:~/repos/docker-pi-hole$ ./build.sh -w kjansdkajnsd
Error: Web branch 'kjansdkajnsd' not found. Exiting.

And from my synology, which does not have buildx...

adam@fappotron:/volume1/copy$ ./build.sh
Error: Docker buildx is required to build this image. For installation instructions, see:
       https://github.com/docker/buildx#installing

And back to the main machine to prove it still works:

adam@adam-pc:~/repos/docker-pi-hole$ ./build.sh -c development -f development -w development -p development
Executing command: docker buildx build src/. --tag pihole:local --load --no-cache --build-arg CORE_BRANCH=development --build-arg FTL_BRANCH=development --build-arg WEB_BRANCH=development --build-arg PADD_BRANCH=development
[+] Building 5.4s (19/19) FINISHED                                             docker:default
 => [internal] load build definition from Dockerfile                                     0.0s
 => => transferring dockerfile: 4.04kB                                                   0.0s
 => resolve image config for docker-image://docker.io/docker/dockerfile:1                0.7s
 => CACHED docker-image://docker.io/docker/dockerfile:1@sha256:865e5dd094beca432e8c0a1d  0.0s
 => [internal] load metadata for docker.io/library/alpine:3.20                           0.6s
 => [internal] load .dockerignore                                                        0.0s
 => => transferring context: 82B                                                         0.0s
 => CACHED [base 1/9] FROM docker.io/library/alpine:3.20@sha256:beefdbd8a1da6d2915566fd  0.0s
 => CACHED [base 5/9] ADD --chmod=0755 https://raw.githubusercontent.com/pi-hole/PADD/d  0.2s
 => CACHED [base 3/9] ADD https://ftl.pi-hole.net/macvendor.db /macvendor.db             0.0s
 => [internal] load build context                                                        0.0s
 => => transferring context: 98B                                                         0.0s
 => [base 2/9] RUN apk add --no-cache     bash     bind-tools     binutils     coreutil  1.2s
 => [base 3/9] ADD https://ftl.pi-hole.net/macvendor.db /macvendor.db                    0.0s
 => [base 4/9] COPY crontab.txt /crontab.txt                                             0.0s
 => [base 5/9] ADD --chmod=0755 https://raw.githubusercontent.com/pi-hole/PADD/developm  0.0s
 => [base 6/9] RUN git clone --depth 1 --single-branch --branch development https://git  1.2s
 => [base 7/9] RUN cd /etc/.pihole &&     install -Dm755 -d /opt/pihole &&     install   0.4s
 => [base 8/9] COPY --chmod=0755 bash_functions.sh /usr/bin/bash_functions.sh            0.0s
 => [base 9/9] COPY --chmod=0755 start.sh /usr/bin/start.sh                              0.0s
 => [remote-ftl-install 1/1] RUN if   [ "linux/amd64" = "linux/amd64" ];    then FTLARC  0.9s
 => exporting to image                                                                   0.2s
 => => exporting layers                                                                  0.2s
 => => writing image sha256:d6d29ec346804c6153ad3794d552e502f020e7a043f76c9e81c90f005a4  0.0s
 => => naming to docker.io/library/pihole:local                                          0.0s

Successfully built Docker image with tag 'pihole:local'
REPOSITORY   TAG       IMAGE ID       CREATED                  SIZE
pihole       local     d6d29ec34680   Less than a second ago   89.9MB