Adapting validation for docker container network mode to include ENV_VARS

[zedfmario]

Description

In PR #5088 I enabled the possibility of reusing the network stack from another docker container.
That mode required either a name or a docker id to check what container you want to reuse the network stack from.

The configuration at that point required the exact name or id. It's quite common in the docker environment, when referencing by id, to use the very first 12 chars of the container name, as those are the ones shown when docker ps is executed.
Example:

> docker ps
CONTAINER ID        IMAGE               COMMAND               CREATED             STATUS              PORTS               NAMES
7765077b5d37        alpine              "tail -f /dev/null"   About an hour ago   Up About an hour                        alpine

This PR finds docker containers by ids using HasPrefix instead of equal. It also adapts the validation for docker container network mode to include ENV_VARS. It will automatically translate the ENV_VAR with the os.Environment variable.

User facing changes (remove if N/A)

from now on, it is possible to write the container name as follows:

build:
  docker:
    network: "container:{{.ENV_VAR}}"

where ENV_VAR could be 7765077b5d37, for example.

[codecov]

Codecov Report

Merging #5589 (0897b85) into master (70cd58b) will decrease coverage by 0.02%.
The diff coverage is 73.07%.

@@            Coverage Diff             @@
##           master    #5589      +/-   ##
==========================================
- Coverage   71.08%   71.06%   -0.03%     
==========================================
  Files         402      402              
  Lines       15145    15182      +37     
==========================================
+ Hits        10766    10789      +23     
- Misses       3586     3598      +12     
- Partials      793      795       +2     

Impacted Files	Coverage Δ
pkg/skaffold/schema/validation/validation.go	86.64% <73.07%> (-3.78%)	⬇️

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[tejal29]

few nits.