A nifty trick when you are entering a password in your terminal:

read -s mypassword
<enter your password>

kubectl create secret docker-registry registry-credentials --namespace nuclio \
    --docker-username <username> \
    --docker-password $mypassword \
    --docker-server <URL> \
    --docker-email ignored@nuclio.io

unset mypassword

man bash

Entry for man bash which explains read:

read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]
              One  line is read from the standard input, or from the file descriptor fd supplied as an argument to the -u option, split into words as described above under Word
              words  and  their  intervening  delimiters are assigned to the last name.  If there are fewer words read from the input stream than names, the remaining names are
              Splitting).   The  backslash character (\) may be used to remove any special meaning for the next character read and for line continuation.  Options, if supplied,
              -e     If  the standard input is coming from a terminal, readline (see READLINE above) is used to obtain the line.  Readline uses the current (or default, if line
                     If readline is being used to read the line, text is placed into the editing buffer before editing begins.
                     read returns after reading nchars characters rather than waiting for a complete line of input, but honors a delimiter if fewer than nchars  characters  are
                     read before the delimiter.
                     read returns after reading exactly nchars characters rather than waiting for a complete line of input, unless EOF is encountered or read times out.  Delim‐
                     iter characters encountered in the input are not treated specially and do not cause read to return until nchars characters are read.   The  result  is  not
                     split on the characters in IFS; the intent is that the variable is assigned exactly the characters read (with the exception of backslash; see the -r option
                     Display prompt on standard error, without a trailing newline, before attempting to read any input.  The prompt is displayed only if input is coming from  a
                     Cause read to time out and return failure if a complete line of input (or a specified number of characters) is not read within  timeout  seconds.   timeout
                     may  be  a  decimal  number with a fractional portion following the decimal point.  This option is only effective if read is reading input from a terminal,
                     pipe, or other special file; it has no effect when reading from regular files.  If read times out, read saves any partial input  read  into  the  specified
                     variable  name.   If  timeout is 0, read returns immediately, without trying to read any data.  The exit status is 0 if input is available on the specified
              If no names are supplied, the line read is assigned to the variable REPLY.  The exit status is zero, unless end-of-file is encountered, read times out  (in  which
              case  the status is greater than 128), a variable assignment error (such as assigning to a readonly variable) occurs, or an invalid file descriptor is supplied as