docs: add custom build instruction for docker

Author: rinarjn

apps/website/docs/contribution/core.md

@@ -31,6 +31,92 @@ $ make prd-mt
 
 The output file locates at **/packages/core** or **/packages/core-mt**.
 
+## Custom Build / Reduce Build Size
+
+You can customize your build to include only the libraries you need, which can significantly reduce the final build size. This is done by modifying the `Dockerfile`.
+
+---
+
+### Step-by-Step Example: Removing WebP Support
+
+Here's how to remove `libwebp` (WebP image support) from the build. The same principle applies to other libraries.
+
+You need to make changes in **4 places** in the `Dockerfile`:
+
+#### 1. Remove the library builder stage
+
+Find the stage that builds the library you want to remove and delete the entire block:
+
+```dockerfile
+# Remove this entire block
+FROM emsdk-base AS libwebp-builder
+COPY --from=zlib-builder $INSTALL_DIR $INSTALL_DIR
+ENV LIBWEBP_BRANCH=v1.3.2
+ADD https://github.com/ffmpegwasm/libwebp.git#$LIBWEBP_BRANCH /src
+COPY build/libwebp.sh /src/build.sh
+RUN bash -x /src/build.sh
+```
+
+#### 2. Remove the COPY instruction
+
+In the `ffmpeg-base` stage, remove the line that copies the built library:
+
+```dockerfile
+# Remove this line
+COPY --from=libwebp-builder $INSTALL_DIR $INSTALL_DIR
+```
+
+#### 3. Remove the configure flag
+
+In the `ffmpeg-builder` stage, remove the corresponding `--enable-lib...` flag:
+
+```dockerfile
+# Remove this line from the ffmpeg-builder stage
+--enable-libwebp \
+```
+
+#### 4. Remove the linker flags
+
+In the `ffmpeg-wasm-builder` stage, remove the library from `FFMPEG_LIBS`:
+
+```dockerfile
+# Remove these lines from FFMPEG_LIBS
+-lwebpmux \
+-lwebp \
+-lsharpyuv \
+```
+
+> **💡 Pro Tip:** Start by removing just the main library flag (e.g., `-lwebp`). If the build fails with "undefined reference" errors, those errors will tell you exactly which additional libraries to remove.
+
+#### 5. Build and test
+
+```bash
+# Run the build command
+make prd
+
+# Output will be in packages/core/dist/
+```
+---
+
+**Additional Build Size Optimization:**
+
+You can sometimes play around with `build/ffmpeg-wasm.sh` and `build/ffmpeg.sh` to disable things you are not using to make the size smaller.
+
+### More Advance Customization Example: Creating a Minimal Build
+
+For more advanced customization, you might want to create a minimal build that only includes the features you need. A good example is creating a build that can create a video from a sequence of images (e.g., from an HTML canvas), handle MP4 encoding/decoding, and support audio.
+
+A community member, @Kaizodo, shared an approach that resulted in a build size of only 4.80MB. You can find the full details and a discussion in [GitHub Issue #866](https://github.com/ffmpegwasm/ffmpeg.wasm/issues/866).
+
+The general strategy is to:
+
+1.  **Start with a minimal configuration:** Instead of removing libraries one by one, a more effective approach is to start with a minimal ffmpeg configuration. This can be achieved by using flags like `--disable-everything` in the ffmpeg configuration step within the `Dockerfile`.
+2.  **Enable specific components:** After disabling everything, you can selectively enable only the encoders, decoders, muxers, demuxers, and protocols you need for your specific use case. For example: `--enable-encoder=libx264`, `--enable-decoder=png`, `--enable-muxer=mp4`, etc.
+3.  **Include only necessary libraries:** Make sure your `Dockerfile` only builds and links the external libraries that correspond to the features you enabled (e.g., `libx264`). You can remove the build stages for any other libraries.
+
+This approach gives you control over the build content and its final size.
+
+We would like to thank @Kaizodo, @harkdawg and other community members for sharing their knowledge!
 ## Publish
 
 Simply run `npm publish` under **packages/core** or **/packages/core-mt**.