Stand in to merge docker compose file merging (docker stack deploy doesn't do the magic yet), does the 'extends' replacement and the env var replacement.

compose-merge.pl

#!/usr/bin/perl
#
#====================================================
#
# Stand in for docker-compose 'extends' and env replacement  
# ( currently not implemented for `docker stack deploy -f` )
#
#====================================================

use strict;
use warnings;
use YAML::Syck qw(LoadFile Dump);

use File::Spec;
use File::Basename;
use Hash::Merge qw(merge);
use Data::Dumper;
use Cwd qw(abs_path);

$YAML::Syck::SingleQuote = 1;

Hash::Merge::specify_behavior(
	{
		'SCALAR' => {
			'SCALAR' => sub { $_[0] },
			'ARRAY'  => sub { $_[0] },
			'HASH'   => sub { $_[0] },
		},
		'ARRAY' => {
			'SCALAR' => sub { [ @{ $_[0] }, $_[1] ] },
			'ARRAY'  => sub { [ @{ $_[1] }, @{ $_[0] } ] },
			'HASH'   => sub { [ @{ $_[0] }, values %{ $_[1] } ] },
		},
		'HASH' => {
			'SCALAR' => sub { $_[0] },
			'ARRAY'  => sub { $_[0] },
			'HASH'   => sub { Hash::Merge::_merge_hashes( $_[0], $_[1] ) },
		},
	},
	'COMPOSE'
);
Hash::Merge::set_behavior('COMPOSE');

my @confFileList = map{ [$_, LoadFile($_)] }@ARGV;

sub parseExtends{

    my $service = $_[0];
    my $serviceMap = $_[1];
    my $dir = $_[2];
    my $extends = $service->{extends};
    if ( exists($extends->{service}))
    {
        my $serviceToMerge = $extends->{service};
        my $extendedServiceDef;
        if (exists($extends->{'file'})) {
            $extendedServiceDef = getExtendedService($extends->{file}, $extends->{service}, $dir);
        } else {
            $extendedServiceDef = $serviceMap->{$extends->{service}};
        }

        delete $service->{extends};
        my $newHash  = merge(  $service, $extendedServiceDef);
        $service = $newHash;
    }
    return $service;
}

#====================================================
#
# Get the hash map for the service def in anotherfile
#
# TODO - recursive call if this extends
#====================================================
sub getExtendedService{

    my $file = $_[0];
    my $serviceToMerge = $_[1];
    my $basePath = $_[2];


    my $resolvedFile = $file;#File::Spec->catfile($basePath, $file);
    my $loaded = LoadFile($resolvedFile);
    my $serviceMap = $loaded->{services};

    if (exists $serviceMap->{$serviceToMerge}->{extends}) {
        $serviceMap->{$serviceToMerge} = parseExtends($serviceMap->{$serviceToMerge}, $serviceMap, dirname($resolvedFile));
    }

    return formatPathArgs($serviceMap->{$serviceToMerge}, dirname($file));

}

sub stripDotDot {

    my @arr;
    my $c = 0;
    for (File::Spec->splitdir($_[0]))
    {
        
        if ($_ ne ".." && $c ne 0) { 
            push @arr, $_ 
        }
        else { 
            pop @arr
        }
        $c ++;
    }
    return File::Spec->catfile(@arr);
}

sub formatPathArgs {
    my $service = $_[0];
    return $service;
    #my $dir =$_[1];
    ##========================
    ## Loop through and replace list of 
    ## vars that are dir paths on host
    ## =======================
    #for my $varName ('env_file') {
        #if (exists $service->{$varName} ) {
            #if(ref($service->{$varName}) eq 'ARRAY') {
                #my @envFileList = $service->{$varName};
                #for my $i  (0 .. $#{ $service->{$varName} }){
                    #my $f = $service->{$varName}->[$i];
                    #my $p = $dir . '/' . $f;

                    #$service->{$varName}->[$i] = $p;
                #}
            #} else {
                #my $p = $dir . '/' . $service->{$varName};
                #$service->{$varName} = $p;
            #}
        #}
    #}
    #return $service;
}

sub replaceEnvs {
    my $data = $_[0];

    #TODO add in fallback parsing ${SOME_ENV:-fallback_value}
    $data =~ s{(\$)?\$+(\w+)}{(defined $1) ? "$1$2" : (exists $ENV{$2}) ? $ENV{$2} : q//}ge;
    $data =~ s{(\$)?\$+\{(\w+)\}}{(defined $1) ? "$1$2" : (exists $ENV{$2}) ? $ENV{$2} : q//}ge;

    return $data;
}

for my $fileHashPair (@confFileList) {

    my $fileName = @$fileHashPair[0];
    my($vol,$dir,$file) = File::Spec->splitpath($fileName);
    my $confMapRef = @$fileHashPair[1];
    my $services = $confMapRef->{services};

    for my $k (keys %$services)
    {
        my $service =$services->{$k};

        if (exists $service->{extends}) {
            $confMapRef->{services}->{$k} = parseExtends($service, $confMapRef->{services}, $dir);
        }
        $confMapRef->{services}->{$k} = formatPathArgs($confMapRef->{services}->{$k}, dirname($fileName));
    }

}


my $totalConfMapRef = merge(reverse map{ @$_[1] }@confFileList);


my $newYaml = Dump($totalConfMapRef);

print "version: '3.4'\n";
my $c = 0;
for (split /^/, $newYaml) {
    if ($_ !~ /^(")?version(")?:.*/ && $c != 0) {
        #$_ = replaceEnvs($_);
        print $_;
    }
    $c ++;
}