Docker image builds itself when starting

[Nospamas]

Outlining the problem:

• When pulling the image the image takes the full javascript source from the repostory
• When deployed to a server, the javascript must be built before the site can be served
• This results in a long delay before the container is usable after starting

Why this is bad:

• I think this is better explained by better writers here: https://12factor.net/disposability
• Building requires a very large spike in resource usage (Multi gigabytes of ram, CPU spike) compared to its normal running scenario. This makes it harder to provide good resource limits if we migrate to k8s

Reasons for the problem (I think):
Create react app requires you to provide environment variables at build time. As such, as there are two different versions of this app (4 if you count dev/prod split) those variables need to be provided at runtime. Because of the CRA limitation, we currently have to build the javascript during initial runtime in order to provide those variables, resulting in the above behaviour.

Potential solutions:
Fixing it isn't trivial, but these might be potential routes:

• one build per environment (not great, we lose customisation post build and would need to store multiple images)
• maybe a bash script? https://blog.risingstack.com/create-react-app-runtime-env-cra/ (potentially good, gives us runtime control, need to be careful of PUBLIC_URL which we do vary and is also stored in index.html not just javascript)
• We are far from alone in the issue, some discussion here: Runtime environment variables facebook/create-react-app#2353

[Nospamas]

a more bash based solution: https://medium.com/@hasniarif/how-to-handle-runtime-environment-variables-with-react-ec809cb07831