Empty lockfiles will trigger lockfile to init/populate

[joshspicer]

This change provides a way to generate a lockfile without passing the --experimental-lockfile flag.

If a user want to "opt-in" to lockfiles, they may create an empty file in the expected location. The next build of the dev container will write the lockfile information.

This is useful for Codespace users who aren't able to directly influence the flags passed to the dev container CLI. For these users, the instructions to "opt-in" to lockfiles could be to touch .devcontainer-lock.json and rebuild their codespace.

Note that if the --experimental-frozen-lockfile flag is passed, an empty lockfile WILL be considered different and the CLI will throw an error.

Today, providing an empty lockfile throws a JSON parsing error:

[1 ms] @devcontainers/cli 0.51.2. Node.js v16.20.0. linux 5.15.0-1041-azure x64.
SyntaxError: Unexpected end of JSON input
    at JSON.parse (<anonymous>)
    at readLockfile (/workspaces/cli/dist/spec-node/devContainersSpecCLI.js:53202:17)
    at async generateFeaturesConfig (/workspaces/cli/dist/spec-node/devContainersSpecCLI.js:54093:20)
    at async getExtendImageBuildInfo (/workspaces/cli/dist/spec-node/devContainersSpecCLI.js:55410:26)
    at async extendImage (/workspaces/cli/dist/spec-node/devContainersSpecCLI.js:55333:30)
    at async buildNamedImageAndExtend (/workspaces/cli/dist/spec-node/devContainersSpecCLI.js:56402:10)
    at async openDockerfileDevContainer (/workspaces/cli/dist/spec-node/devContainersSpecCLI.js:56336:19)
    at async resolveWithLocalFolder (/workspaces/cli/dist/spec-node/devContainersSpecCLI.js:57198:14)
    at async launch2 (/workspaces/cli/dist/spec-node/devContainersSpecCLI.js:57252:18)
    at async doProvision (/workspaces/cli/dist/spec-node/devContainersSpecCLI.js:60128:20)
{"outcome":"error","message":"Unexpected end of JSON input","description":"An error occurred setting up the container."}

[chrmarti]

Good idea, thanks!