libpng-dev dependency

[StefanPahlplatz]

In order to build the project (at least on ubuntu 18.04 with the minimal install option) you need to manually install libpng-dev. Otherwise you get the following error:

yarn install v1.9.4
[1/4] Resolving packages...
[2/4] Fetching packages...
info fsevents@1.2.4: The platform "linux" is incompatible with this module.
info "fsevents@1.2.4" is an optional dependency and failed compatibility check. Excluding it from installation.
[3/4] Linking dependencies...
warning " > gatsby-plugin-react-next@2.0.1-13" has incorrect peer dependency "gatsby@^1.0.0".
warning "gatsby-plugin-sass > sass-loader@7.0.3" has unmet peer dependency "webpack@^3.0.0 || ^4.0.0".
warning " > gatsby-remark-images@2.0.1-beta.9" has incorrect peer dependency "gatsby-plugin-sharp@^2.0.0-beta.5".
warning " > gatsby-transformer-sharp@2.1.1-beta.6" has incorrect peer dependency "gatsby-plugin-sharp@^2.0.0-beta.3".
warning " > bootstrap@4.1.3" has unmet peer dependency "jquery@1.9.1 - 3".
warning " > bootstrap@4.1.3" has unmet peer dependency "popper.js@^1.14.3".
[4/4] Building fresh packages...
[1/5] ⡀ sharp
[2/5] ⡀ node-sass
[3/5] ⡀ pngquant-bin
[4/5] ⡀ cwebp-bin
error /home/stefan/Projects/gatstrap/node_modules/pngquant-bin: Command failed.
Exit code: 1
Command: node lib/install.js
Arguments: 
Directory: /home/stefan/Projects/gatstrap/node_modules/pngquant-bin
Output:
⚠ The `/home/stefan/Projects/gatstrap/node_modules/pngquant-bin/vendor/pngquant` binary doesn't seem to work correctly
  ⚠ pngquant pre-build test failed
  ℹ compiling from source
  ✔ pngquant pre-build test passed successfully
  ✖ Error: pngquant failed to build, make sure that libpng-dev is installed
    at Promise.all.then.arr (/home/stefan/Projects/gatstrap/node_modules/pngquant-bin/node_modules/bin-build/node_modules/execa/index.js:231

[jaxx2104]

Please retry package install.

rm -rf node_modules .cache
yarn install