Add support for escaping $ in envsubst deployer

[huyhg]

We should have an environment variable named DOLLAR, and the template can write something like this to output literal `$':

${DOLLAR}MY_RAW_VAR

[khajduczenia]

Could we consider another way of escaping '$' in manifest templates? The convention with '${DOLLAR}' strongly affects the readability of scripts and configuration files for the developers.

I think we could improve the situation by:

1. using a common escaping pattern of '$' in the manifest files
2. in the deployer script:

• replace all the occurrences of '$' with '${DOLLAR}',
• export DOLLAR="$",
• use envsubst just as before.

This way, we have pretty readable manifest files, using a common escaping convention with just a bit more complicated deployer script.

[khajduczenia]

It looks my escape character got escaped in my message - in points 1. and 2., wherever there is '$' it should be '\$'.

[huyhg]

Agreed that ${DOLLAR} is hard to read. We should do something else.

I would like to avoid manual search and replace if possible--it's hard to get it right in all corner cases.

@trironkk Do you have any thoughts?

[huyhg]

There's this library that supports such functionality. We can make a wrapper out of it.

However, doing anything custom makes us farther away from enabling users to expand manifests and deploy the app using CLI.

[deustis]

We might consider adding a new deployer base image that uses gotpl instead of envsubst. The advantage there is being able to pass a values.yaml and perhaps also do some slightly fancier expansion logic.