Allow to set ENV vars for build docker and deploy kubernetes

[aguschin]

this is for #647 (comment)

This work similarly to #645:

mlem build docker works the same, but I'll provide an example for k8s deploy here:

$ mlem declare deployment kubernetes deployer \
  --image_name myimage --service_type loadbalancer --registry remote \
  --env docker --env.registry remote --registry.host localhost --namespace myns \
  --set_env.0 VAR1 --set_env.1 VAR2=aguschin
💾 Saving deployment to deployer.mlem

# deployer.mlem
env:
  object_type: env
  registry:
    type: remote
  type: kubernetes
image_name: myimage
namespace: myns
object_type: deployment
registry:
  host: localhost
  type: remote
service_type:
  type: loadbalancer
set_env:
- VAR1
- VAR2=aguschin
type: kubernetes

Then in Dockerfile for your image you'll get:

...
ENV VAR1=VALUE1
ENV VAR2=aguschin

VALUE1 will be taken from shell/env vars with os.getenv("VAR1"). If it's not set, an Exception will be raised by MLEM.

[codecov]

Codecov Report

Patch coverage: 51.85% and project coverage change: -0.07 ⚠️

Comparison is base (4c5be67) 85.77% compared to head (d0bf40a) 85.70%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #648      +/-   ##
==========================================
- Coverage   85.77%   85.70%   -0.07%     
==========================================
  Files         109      109              
  Lines        9904     9921      +17     
==========================================
+ Hits         8495     8503       +8     
- Misses       1409     1418       +9     

Impacted Files	Coverage Δ
mlem/contrib/docker/base.py	85.00% <ø> (ø)
mlem/contrib/flyio/meta.py	46.08% <0.00%> (-1.67%)	⬇️
mlem/contrib/kubernetes/build.py	100.00% <ø> (ø)
mlem/core/errors.py	83.33% <50.00%> (-1.67%)	⬇️
mlem/contrib/docker/context.py	80.41% <56.25%> (-0.59%)	⬇️
mlem/contrib/kubernetes/base.py	85.84% <100.00%> (+0.27%)	⬆️

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