Add some information about building the docker image locally

docs/docker/build-image.md

@@ -0,0 +1,79 @@
+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 convenient `build.sh` script located in the root of the [docker-pi-hole repository](https://github.com/pi-hole/docker-pi-hole)
+
+## Checking out the repository
+
+In order to build the image locally, you will first need a copy of the repository on your computer. The following commands will clone the repository from Github and then put you into the directory
+
+```
+git clone https://github.com/pi-hole/docker-pi-hole
+cd docker-pi-hole
+git checkout development #NOTE: This step is only needed until V6 is released
+```
+
+All other commands following assume you have at least run the above steps.
+
+## Build.sh
+
+```
+Usage: ./build.sh [-l] [-f <ftl_branch>] [-c <core_branch>] [-w <web_branch>] [-t <tag>] [use_cache]
+Options:
+  -f, --ftlbranch <branch>     Specify FTL branch (cannot be used in conjunction with -l)
+  -c, --corebranch <branch>    Specify Core branch
+  -w, --webbranch <branch>     Specify Web branch
+  -p, --paddbranch <branch>    Specify PADD branch
+  -t, --tag <tag>              Specify Docker image tag (default: pihole:local)
+  -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:
+  docker buildx build src/. --tag pihole:local --load --no-cache
+```
+
+## Example uses of the script
+
+### Contributing to the development of `docker-pi-hole`
+
+When contributing, it's always a good idea to test your changes before submitting a pull request. Simply running `./build.sh` will allow you to do so.
+
+There is also `./build-and-test.sh`, which can be used to verify the tests that are run on Github pass with your changes.
+
+```
+git checkout -b myNewFeatureBranch
+#make some changes
+./build.sh
+```
+
+### As an alternative to `pihole checkout`
+
+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.
+
+#### Examples
+
+- You have been asked by a developer to checkout the FTL branch `new/Sensors`. To do so
+
+```
+./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 checkout fix/logRotate
+./build.sh
+```
+
+## Using the built image
+
+Unless otherwise named via the `-t` command, the script will build an image locally and tag it as `pihole:local`. You can reference this as a drop-in replacement for `pihole/pihole:latest` in your compose file or your run command:
+
+```yaml
+services:
+  pihole:
+    container_name: pihole
+    image: pihole:local
+...
+```
+
+```
+docker run [options] pihole:local
+```

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