Run bundle lock --update.
I found an answer in a blog post by Chris Blunt: “Rails on Docker: Quickly Create or Update Your Gemfile.lock”:
Today, I discovered a way to save the hours wasted downloading gems: bundler’s lock command.
This gem of a command resolves your app’s dependencies and writes out the appropriate
Gemfile.lock– without installing any of the gems themselves.
According to the changelog, this command was added in Bundler 1.10.0.pre, released about eight months after this question was asked.