fix: do not error if failed to push image

[johnstcn]

Relates to #385

This adds an option ENVBUILDER_EXIT_ON_PUSH_FAILURE that controls the previous behaviour of exiting with an error if ENVBUILDER_PUSH_IMAGE was set to a truthy value and we failed to push the image.

Before, Envbuilder would always exit with an error on a failed push.
Now, Envbuilder will only exit with an error if ENVBUILDER_EXIT_ON_PUSH_FAILURE is set to a truthy value. Otherwise, Envbuilder will continue building the image.

#3: Pushing image to localhost:5000/cache
#3: ⚠️️ Failed to push image! [1.835942ms]
#4: 🔄 Updating the ownership of the workspace...
#4: 👤 Updated the ownership of the workspace! [406.355µs]
#5: 🔄 Updating ownership of /home/coder...
#5: 🏡 Updated ownership of /home/coder! [56.162µs]
=== Running init command as user "coder": ["/bin/sh" "-c" "bash"]
🐳 bedd93b40e11 envbuilder-starter-devcontainer # whoami
coder

[johnstcn]

Almost done here, just need to get the test for this working properly.

EDIT: I ended up choosing a different solution. My original idea was to expose an environment variable with the error resulting from a push, but that ended up being a less intuitive and more complex change than just adding a companion option to ENVBUILDER_EXIT_ON_BUILD_FAILURE.

[SasSwart]

LGTM! Glad we have this now.