feat: add variable expansion in manifest.build.dockerfile

[jLopezbarb]

Proposed changes

This PR adds support for variable expansion on the expanded syntax of build.Dockerfile and build.Context

How to validate

1. Create the following manifest:

build:
  app:
    context: .
    dockerfile: $DOCKERFILEPATH

1. Run the command export DOCKERFILEPATH=Dockerfile
2. Run okteto build

CLI Quality Reminders 🔧

For both authors and reviewers:

• Scrutinize for potential regressions
• Ensure key automated tests are in place
• Build the CLI and test using the validation steps
• Assess Developer Experience impact (log messages, performances, etc)
• If too broad, consider breaking into smaller PRs
• Adhere to our code style and code review guidelines

[codecov]

Codecov Report

Merging #4217 (9d6b475) into master (27e1d9c) will decrease coverage by 0.01%.
Report is 6 commits behind head on master.
The diff coverage is 33.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4217      +/-   ##
==========================================
- Coverage   45.64%   45.63%   -0.01%     
==========================================
  Files         298      298              
  Lines       27262    27266       +4     
==========================================
  Hits        12443    12443              
- Misses      13762    13764       +2     
- Partials     1057     1059       +2     

[maroshii]

Code changes LGTM based on how we currently do variable expansion.

As a side comment and for future reference, I don't think it scales to change these upon request. Why would we expand Dockerfile and Context and not Target and Image? I would expect a parser to generically expand every field generically

[maroshii]

Will this need documentation?

[ifbyol]

@jLopezbarb Do we need to backport this to release-2.26?