Add some information about building the docker image locally

Signed-off-by: Adam Warner <me@adamwarner.co.uk>

docs/docker/build-image.md

@@ -0,0 +1,66 @@
+In case you wish to customise the image, or perhaps check out a branch after being asked by a developer to do so, you can use the convienient `build.sh` script located in the root of the [docker-pi-hole repository](https://github.com/pi-hole/docker-pi-hole)
+
+#### Usage:
+
+```
+./build.sh [-l] [-f <ftl_branch>] [-c <core_branch>] [-w <web_branch>] [-t <tag>] [use_cache]
+```
+
+#### Options:
+
+```
+ `-f <branch>` /  `--ftlbranch <branch>`: Specify FTL branch (cannot be used in conjunction with `-l`)
+ `-c <branch>` / `--corebranch <branch>`: Specify Core branch
+ `-w <branch>` / `--webbranch <branch>`: Specify Web branch
+ `-t <tag>` / `--tag <tag>`: Specify Docker image tag (default: `pihole`)
+ `-l` / `--local`: Use locally built FTL binary (requires `src/pihole-FTL` file)
+ `use_cache`: Enable caching (by default `--no-cache` is used)
+```
+
+If no options are specified, the following command will be executed, and an image will be created based on the current branch of the repository, and all `master` component branches.
+
+```
+docker buildx build src/. --tag pihole --no-cache
+```
+
+Once the command has run, a local image will have been created named `pihole`, you can reference this as a drop-in replacement for `pihole/pihole:latest` in your compose file or your run command:
+
+```yml
+services:
+  pihole:
+    container_name: pihole
+    image: pihole
+...
+```
+
+```
+docker run [options] pihole
+```
+
+Then start your container as normal.
+
+### `pihole checkout` alternative
+
+Occasionally you may need to try an alternative branch of one of the components (`core`,`web`,`ftl`). On bare metal you would run, for example, `pihole checkout core branchName`, however in the Docker image we have disabled this command as it can cause unpredictable results.
+
+The preferred method is to use the script documented above
+
+#### Examples
+
+- You have been asked by a developer to checkout the FTL branch `new/Sensors`. To do so
+
+```
+git clone https://github.com/pi-hole/docker-pi-hole
+cd docker-pi-hole
+git checkout development-v6 # NOTE: This step is only needed until V6 is released
+./build.sh -f new/Sensors
+```
+
+- There is new docker-specific work being carried out on the branch `fix/logRotate` that you wish to test
+
+```
+git clone https://github.com/pi-hole/docker-pi-hole
+cd docker-pi-hole
+git checkout fix/logRotate
+./build.sh
+```

mkdocs.yml

@@ -158,6 +158,7 @@ nav:
     - "Pi-hole extensions": regex/pi-hole.md
     - "Approximate matching": regex/approximate.md
   - 'Docker':
+    - 'Building': docker/build-image.md
     - 'DHCP': docker/DHCP.md
   - 'Contributing':
     - 'Contributing on GitHub': guides/github/contributing.md