How can I add custom '.desktop' application launchers to Ubuntu Dock?

1. Create a .desktop file (or, copy if you already have one ready), say my-chrome.desktop, in your ~/.local/share/applications/ directory.
   You may add the following lines to it for a simple, bare-minimum .desktop launcher

   [Desktop Entry]
   Name=My Chrome
   Comment=Chrome my profile
   Exec=google-chrome --profile-directory=Default
   Icon=google-chrome
   Type=Application
   Terminal=false
   

   The keys are as described below

   • Name= Specific name of the application (For this particular example, I used "My Chrome" to avoid confusion with the already existing Google Chrome launcher. You can use any other name.)
   • Comment= A short description of the application
   • Exec= The command to execute in order to launch the application
   • Icon= Icon for the application (You can use an icon provided by your icon theme or an absolute path to an image file.)

2. Click 'Activities' or 'Show Applications' and search for "My Chrome" (application name as per the Name key). It should appear.

3. Right click on it and select 'add to favourites'.
   If right clicking on application icon in dock doesn't show "Add to favourites" option, search for the application in Activities screen, and then drag it across to the dock (suggested by Legolas).

If nothing works, see this Q&A: Cannot add custom launcher to Ubuntu Dock (Add to Favorites)

I had the same problem with Firefox Nightly. The following configuration works for me in ~/.local/share/applications/firefox-nightly.desktop:

[Desktop Entry]
Type=Application
Name=Firefox Nightly
#
# WARNING: Remember to fix the path in Icon and Exec
#
Icon=/path/to/firefox-nightly/browser/icons/mozicon128.png
Exec=/path/to/firefox-nightly/firefox %u
Terminal=false
StartupWMClass=Nightly

Without StartupWMClass, the running application is not recognized and the icon is displayed twice on the dock.

For anyone having troubles, if the application is not appearing in Activities, make sure you don't have a

NoDisplay=true

line in the .desktop file for your application.