bash script - sed - template file processing
You are parsing a JSON file using some UNIX tools. This is risky, since those can have so many edge cases that you will end up refactoring over and over.

For example, you mention that sed s/\\[Placeholder.A]/${MY_VALUE}/ is giving errors when $MY_VALUE has slashes. This is normal, since this is how it gets expanded:

sed s/\[Placeholder.A]/${MY_VALUE}/
                           ↓    
sed s/\[Placeholder.A]/http://aaa.com/
#    ^                ^     ^^       ^


See? Too many slashes for a simple sed 's/find/replace/, thus it fails. You can solve this by using another separator:

$ sed 's/\[Placeholder.A\]/a/' file
{
    "networkMode": "bridge",
    "name": "a"     
}
$ MY_VALUE="http://aaa.com"
$ sed "s/\[Placeholder.A\]/$MY_VALUE/" file
sed: -e expression #1, char 27: unknown option to `s'

# Let's use ? as a separator, for example
$ sed "s?\[Placeholder.A\]?$MY_VALUE?" file
{
    "networkMode": "bridge",
    "name": "http://aaa.com"     
}




Instead, use jq, a command-line JSON processor. Given your file:

$ cat file
{
    "networkMode": "bridge",
    "name": "[Placeholder.A]"     
}


See how easy is to replace the value in the "name" key:

$ jq '.name="a"' file
{
  "networkMode": "bridge",
  "name": "a"
}


If you want to use a variable then you will need to do some quotes-opening-closing:

$ MY_VALUE="http://aaa.com"
$ jq '.name="'$MY_VALUE'"' file
{
  "networkMode": "bridge",
  "name": "http://aaa.com"
}